I'm adapting the following from an older answer of mine, but with some additional material as well.

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I know most about the British experience, so I'll be sticking to that below.

First, here is the standard issue daily food ration for a British soldier in the summer of 1917:

Item	Amount
Meat (fresh or frozen)	1 lb
OR Meat (preserved) Bully Beef	9 oz
Bread	1 lb
OR Biscuit	10 oz
Bacon	4 oz
Cheese	2 oz
Fresh Vegetables	8 oz
OR Dried Vegetables	2 oz
Tea	5/8 oz
Jam (plum and apple)	3 oz
Butter	2 oz
Sugar	3 oz
Oatmeal (3 times a week)	2 oz
Rice	1 oz
Salt	1/4 oz
Mustard	1/100 oz
Pepper	1/100 oz
Milk (condensed)	1 oz
Pickles (weekly)	1 oz

Source: Andrew Robertshaw's Feeding Tommy (2013; p.68)

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Now, let's talk about distribution -- we'll start with breakfast. Getting hot food in the front lines was amazingly difficult, though the men often found a way. The official means was to have it brought up from the rear by other soldiers carrying huge covered trays ("dixies") of pre-prepared food. Unofficially, many soldiers invested in small portable stoves or enthusiastically burned whatever wasn't tied down, army property or otherwise. Whatever the case, the men could hope for eggs, sausage, and very low-quality toast, if hot food was available; if not, they'd have to settle for hard biscuit and tins of corned beef ("bulley beef") and/or jam.

Tea would be brewed in the trenches, though often only "tea" in the loosest sense of the word; a popular alternative was colloquially called "burgoo," which saw the meager tea stores supplemented by sweetened condensed milk, whatever sugar could be had, and toffee or chocolate from the care packages sent to the men from time to time. The result was a very thick, hot, sweet mess -- perfect for an added dash of rum, a daily ration of which was also typically given out. This often had to be done surreptitiously, though.

When off-duty, soldiers could snack throughout the day on whatever they had at hand. Depending on where they were stationed in the lines, there was a great deal of scope for trade, purchase, or outright thievery, all the way up to liberating entire cows or pigs from their undeserving French or Belgian owners -- but you were asking about rations.

As the day wound down, dinner would be served, and it would often be very similar in its contours to breakfast. More hot food would be brought forward if possible (sausage and eggs again, typically), but would now be ideally supplemented with tinned vegetable stew. Maconochie's was a widely-distributed brand, and that name came to be used as a shorthand for tinned stews of all sorts. Typically lots of root vegetables in a broth. The men would do all they could to make their food more interesting, though often with limited success. It was sometimes possible to procure better fare from civilian sectors behind the lines, but they had it very hard as well and often relied upon the presence of the soldiers to bolster their own diets -- especially when it came to meat. Bully beef was the only meat readily available for some impoverished French and Belgian citizens during the war, and the tins became something of a de facto currency. Certain enterprising ladies even lit their red lanterns over signs offering a whole array of services carefully measured out in quantities of corned beef.

In addition to all of this, there was, as I mentioned above, the rum ration. Rum was drawn by battalion quartermasters in brown earthenware jars marked "SRD," which stood for "Special Rations Department." The men joked that it instead meant "Seldom Reaches Destination" or "Service Rum Diluted." The ration would be distributed out of these jugs, and typically amounted to a quarter gill per man per day -- roughly equivalent to 1/16th of a pint, or about what you'd get in a shot glass. The ration was given out at the morning stand-to, and was officially viewed as "medicine" for political reasons. It was a punishable offense to be found drinking on duty otherwise, but many men found a way and, if they were punished at all, it was often kept "in-house" for the sake of convenience. Nobody likes paperwork.

Now, it would be easy to think that the men in the trenches would starve on such poor fare as described above, but in most cases it was rather the opposite. The ration-production industry was a vast and largely efficient one, and the typical British fighting man was kept up on over 4000 calories a day. The rationing situation was so gigantic that a great deal of wastage sometimes occurred.

From Richard Holmes' Tommy: The British Soldier on the Western Front, 1914-1918 (2004), quoting Captain Gerald Burgoyne:

The wastage in rations is terrible. Going round the place [Kemmel, behind the Ypres salient] this morning, I saw enough tins of Bully thrown away, or simply left in their boxes, to feed a Battalion. The men really get far too much, far more than they can eat, and extra rations like tinned salmon are counted extra and the same amount of Bully is issued to the men. The same with the biscuits. Men prefer to buy bread, and tins after tins of biscuits are lying around everywhere...

Holmes goes on to note an "an artillery headquarters where the tent floors and paths were composed entirely of full tins of bully used as paving stones."

From Corporal Ronald Ginns of the Royal Engineers:

I saw whole cheeses thrown to the pigs, and paths made of bully beef tins unopened, while many a good fire was obtained from army biscuits. But it was the sameness that caused us to spend our pay on French food.

In any event, the "sameness" that Corporal Ginns notes above is at the heart of the matter. British troops were very well-provisioned indeed, but the realities of mass-production led to a great deficit in quality compared to what they could get from the nearest French estaminets or what they could have mailed to them from back home. Since there was no danger of the supply of bully beef and biscuit drying up, these rations were often treated with reckless abandon -- up to and including, as we saw above, being used as architectural supplements.

Sometimes you got more even when you didn't need it to begin with. Captain Burgoyne notes how extra rations were counted as such and the typical rations still came in regardless. This sometimes had interesting consequences in other ways: when a battalion in the firing line was engaged in fighting, rations for the battalion's full complement would be sent forward each day regardless of how many were left alive to receive them. In particularly hot times, this led to each survivor receiving food and drink meant for two or three men, if not more -- and while it was technically an offense to consume rations not intended for you, this was not something that anyone seemed to have the heart to prosecute.

As to your question about the men in reserve or resting behind the lines, there were indeed differences to note.

For reasons that are likely obvious, the most successful field kitchens were those that were situated as far from the firing line as possible. They tended to create a good deal of smoke in spite of the best efforts to construct smokeless stoves (which did achieve some success, I must admit), and consequently became prime morale-damaging targets for enemy artillery or aerial strafing. The British consequently perfected mobile kitchens that could move quickly, deploy quickly, and pack up again in time to get in, cook a passable meal before being bracketed, and get out again. Even at that, though, the food itself had to be brought forward via the dixie system already described above.

Further back, though, the construction of more permanent kitchens was possible -- to say nothing of commandeering the facilities available in still-standing buildings. Those nearest the kitchen could expect to rely on hot and plentiful food, and those working in the kitchens were also not above treating themselves to some choicer cuts of meat when it was available.

An interesting feature of the British Army's cooking discipline was that field cooks were expected to be financially successful. In addition to providing food to be offered as rations to the men, cooks were expected to prepare surplus food for sale to anyone willing to buy it. Each cook was also expected to make his own arrangements with farmers, merchants, and other suppliers to secure a constant supply of fresh (or, often, "fresh enough") ingredients, and cooks developed ingenious means of wringing the most out of what they had to work with. There was a tremendous trade in grease and dripping, for example -- a commodity in which the Army itself had considerable interest (and for which it had considerable paperwork), but which was produced in such quantities that cooks made arrangements to barter it with civilians for more mundane supplies. Some cooks became very wealthy indeed through arrangements such as these -- and sometimes the men under their care even benefited as well.