usually tax rate would be between 1-3% on your wealth, but higher in wartime. sometimes there would be an income tax in Urban areas.

in Rural areas where organised tax collection was either impossible or poorly executed Rome would put a levy and lump sum tax on the provincial governor, as long as he paid and supplied the soldiers needed, be basically got free reign over the region.

http://www.jstor.org/stable/299558 http://www.unrv.com/economy/roman-taxes.php

Some of the other answers here have mentioned the tax rates (although there was no fixed rate at all, and it depended on the province--in Spain and Africa the tributum soli was paid at a fixed rate, regardless of the actual population or wealth of the province, and Asia paid a sort of bizarre early form of the tributum as laid out by the Lex Sempronia) and the various kinds of provincial tributa, but no one has mentioned the vectigals or the method of tax collection, which is what is so unique about Roman tax collection. By the late Republic the vast majority of Roman state revenue came from provincial tribute, and citizens were exempt from the tributum from 167 on. Most citizens wouldn't have to pay taxes at all, and colonies were also generally exempt as they had some form of Italian or Latin status. Any taxes that citizens had to pay would be some form of the vectigalia, the vectigals. By Cicero's day this had become a catch-all phrase for the state's revenue in general, but it appears to have originated as a term describing import/export taxes, as it's derived from "veho," "to convey" (from which we get the word "vehicle"). The vectigals were duties imposed on particular goods and privileges, particularly the portoria, an import/export tax, and the various rents paid on the ager publicus and the public pastures. They were not, like the tributum, levied per head, and therefore citizens who weren't participating in any of the various activities included as vectigals (again, a difficult word, as tribute was often simply lumped in under the term after it came to mean "state revenue") didn't pay any taxes at all. This was perfectly satisfactory--the income earned from the provinces was more than enough to sustain the state, and since magistrates generally relied at least in part on their private funds there was no shortage of cash

However, no discussion of the Roman tax system is complete without mention of the publicans, whom all the other answers have inexcusably left out. The Roman state did not collect taxes itself. That's extremely important, because it's the fundamental principle of the Roman tax system. Originally, back when Roman citizens still paid tributa, the state itself collected that revenue, but it appears that the vectigal was always collected by the publicans. The publicani were private individuals to whom the rights to collect taxes were sold. By the late Republic the publicans collected pretty much all taxes, including the vectigals in Italy. A few taxes, such as the Spanish and African stipendium, collected by the quaestors, and the Asian tithe (that bizarre tax provided by the Lex Sempronia I mentioned earlier), which was actually sold by the censors, were collected by magistrates, but these laws generally applied to provinces which, like Spain and Africa, had existed before the Romans really had many provinces. By Caesar's day provincial management had swelled to a size that the state, particularly the senate and the censors (no doubt made worse by the fact that the censorship basically ceased to exist in the 1st Century, B.C.), simply could not keep up with affairs, and the publicans basically take over. The publicans collected taxes on the mines, the ports, the ager publicus (these three in Italy as well as elsewhere), and in some provinces the various forms of tributum--the precise details of publican affairs is pretty complicated, and with some provinces we don't know precisely how taxes worked. It was an extremely lucrative business, more or less monopolized by the equites, who were often used synonymously with the publicans. Publican companies were large and well-organized, and in general the state sold contracts to the publicans based on how much they expected to earn--the publicans would promise a certain amount (usually by bids, with the highest bidder being the recipient of the contracts), and then whatever they earned beyond that was theirs to keep. Publicans were therefore notoriously extortionist, and their bands of thugs were often so exploitative and menacing that many promagisterial governors found it easier and safer to simply go along with it, rather than try to protect the provincials. Many governors held shares in the companies--although magistrates, senators, and promagistrates were legally barred from holding publican contracts and from holding a share in the company, by the late Republic a market for partes, the unregistered shares of a publican company, had developed, and thus both Caesar and Crassus held large stakes in various publican companies. From an early date they were absurdly wealthy--after Cannae the state's funds were artificially re-created by donations from the publicans from out of their private fortunes. It was also a relatively risky job--an overly-optimistic bid could cost you everything. Famously during Caesar's consulship in 59 he reorganized the contracts that particular companies of publicans, who were almost certainly connected to Crassus, held, since they had bid in excess of what they had discovered they could actually produce--if such a thing occurred the publican would have to pay the difference himself, and it could destroy him if his bid had been unwise. But Cicero described them as the foundation of the state's revenues, and therefore of the state's power itself. The civil wars cost the publicans dearly, as the opposing generals collected revenue themselves. Augustus reformed the provincial taxes, restricting publicans to the collection of the vectigal proper--henceforth tributum would be collected by the quaestors and procurators (who could be just as exploitative as the publicans, and as the OCD notes the exploitation of promagistrates and their staff--who could collect their own duties, particularly grain taxes to feed their troops, which were often embezzled--was generally much in excess of what the publicans could do)

In addition to Batbuckleyourpants answer (the link he provided is great, read it! It goes more into detail):

Taxation in the provinces was hard for 2 main reasons. First there was almost no reliable census data. This is also what Barbuckleyourpants' link mentions. But mainly, the problem was that there just was no formalised administration like we know it today. There was no IRS of the roman republic. Everything was taken care of by the ruling officials and a small circle of personal friends and employees. There were no bureaucrats in the modern sense. The roman republic was just not capable of taxing their provinces! That is also the reason they had to rely on Publicani, private entrepeneurs who bought the right collect taxes in a certain area from the state and did so with help of the local governor (of course enriching themselves by charging extremely high tolls and tariffs and such).

Also, the taxes you had to pay greatly depended on if you were a citizen or not. Roman citizens did not have to pay most of the direct taxes (for example the "tributum capitis", basically a wealth tax), they did not have to pay taxes on property ("tributum soli") as long as it was on the territory of roman cities, and many more exemptions. They did however have to pay a 5% inheritance tax. Taxes that everybody had to pay equally were tolls and tariffs, a 1% sales tax (3% if slaves were sold), and taxes on liberation of your slaves (5% of the value you bought him for).

The tax system changed a lot Diocletian and Constatines reforms were for better or worse but changed the game for Europe