We know of various tribes, but perhaps the most famous tribe that settled in Poland was the Sarmatians.
The Romans knew of them, sometimes fought against them, depicted captive Sarmatians on their coins, hired some as mercenaries, and learned from them, but I doubt that the Romans knew a great deal about the region.
I would like to quote at length:
WHO WERE THE SARMATIANS?
R.Brzezinski, M.Mielczarek
THE SARMATIANS were not a unified people, but rather a number of groups of nomad peoples of similar stock, who wandered generally westwards over the Eurasian steppe - the vast corridor of grasslands, hundreds of miles wide and some 5,000 miles long, extending from China to the Hungarian Plain. They spoke an Iranian language similar to that of the Scythians, and closely related to Persian.
The Sarmatians emerged in the 7th century BC in a region of the steppe to the east of the Don River and south of the Ural Mountains. For centuries they lived in relatively peaceful co-existence with their western neighbours, the Scythians. Then, in the 3rd century BC or slightly earlier, they spilled over the Don to attack the Scythians on the Pontic steppes to the north of the Black Sea (Pontus Euxinus), and 'turned the greater part of the country into a desert' (Diodorus 2.43). The surviving Scythians fled westwards and sought refuge in the Crimea and Bessarabia, leaving their pasturelands to the incomers. The Sarmatians were to dominate these territories over the next five centuries.
The best known of the Sarmatian peoples were the Sauromatae, Aorsi, Siraces, lazyges and Roxolani. The Alans were essentially of the same Iranian stock as the Sarmatians, but are often considered a distinct people. These groupings were tribal confederations rather than individual ethnic tribes; indeed, Ammianus Marcellinus (31.2.13-17) and medieval Arab sources state specifically that the Alans were a coalition of different peoples.
Most Sarmatians were nomads whose grazing herds provided much of the food and clothing they required. They wintered on the southern fringes of the Russian steppe, close to the Black and Caspian Seas and Russia's great rivers, heading north for pasture in the spring. Accompanying them were their covered wagons which doubled as homes - Ammianus Marcellinus notes (31.2.18): “In them husbands sleep with their wives - in them their children are born and brought up”.
The early Sarmatians are now generally regarded as the reality behind the myth of the Amazons. According to Herodotus (4.116), women of the Sauromatae hunted, shot bows and threw javelins from horseback, and went to war dressed in the same clothing as men. This has been confirmed by archaeology: early Sarmatian female graves often contain bronze arrowheads, and occasionally swords, daggers and spearheads; while skeletons of girls aged 13 and 14 have bowed legs - evidence that, like boys, they were often in the saddle before they could walk. The status of women was so unusual that some writers (Pseudo-Scylax, 70) believed that women ruled Sarmatian society.
During the 1st century AD the Sarmatians and Alans truly began to enter recorded history when they conducted a series of spectacular raids on their civilised neighbours. Pouring into Asia Minor, they spread devastation among the Parthians, Medians and Armenians. At the same time other Sarmatian groups ravaged Rome's Danubian provinces of Pannonia and Moesia, before pushing their way along the lower Danube and into the Hungarian Plain to establish a more permanent presence. Some took up military service with the Romans, but for centuries Sarmatians remained unpredictable neighbours, starting wars at the slightest provocation. The pressure was so great that the Romans eventually allowed many to settle within the empire. It was largely as a result of the Sarmatian wars that the Roman army began to abandon its reliance on the legionary infantry and develop an effective cavalry arm - for which the lance-armed Sarmatian cavalry were to provide one important model.
This is really hard to say.
We know that east of the rhine, there where many germanic people. It is very lickly that the germanic tribal reach was far into the east. How far is hard to say, but reaching poland is quite likey.
I normally dont like to link to wikipedia but there are good original illustrations on this: http://en.wikipedia.org/wiki/Germania.
As somebody else allready notted, in the south ruissa, you always have diffrent stepp tribes, generally you can not clearly say who is witch tribe because its not fixed. We generally go by language, during roman times, until the huns, we had iranian speaking stepp nomades. After them the Huns who are kind of hard to put in camp or another. Then during the 6-7 century you have Avars who where probebly turkic (or proto-turkic) speaking people.
There are also the slavs to be consider who might have devloped during that time in eastern european woodlands. There is debate on how and when, that I do not pretent to understand. The best I can give you on this is http://en.wikipedia.org/wiki/Slav#Homeland_debate
So if I jumped back to the year 0 poland I would put my money on meeting some germanic tribesman first.
As for what the romans new. The new quite a bit about the stepp nomads. There is lots of interaction with them specially once the conquer greece who have been in contact with the stepp for a very long time. The knew less about the germanic people, as long as they stayed in south france there contact was limited. Once they are in north france and in the regions of modern day germany you have more and more germanic people and less of the gauls. From then on the needed to have some intrest in controlling or conquering all these germanic tribes. You can read roman source on the diffrent tribes and tribal leaders the romans faced over time.
Apparently the notion that Poles are descended from Sarmatians has become controversial.
The best evidence is probably from the burial mounds, but one easily accessible source is:
http://www.kroraina.com/sarm/jh/index.html
In particular, the passage at:
http://www.kroraina.com/sarm/jh/jh1_2.html
reads in part:
Strabo thus enumerates four Sarmatian tribes: the Iazyges, Royal Sarmatians, Urgi and Roxolani and according to his description, their location on the whole might be conjectured thus: the Iazyges, the Urgi and Royal Sarmatians between the Dnieper and the Danube, furthermore, according to Strabo's description, the southernmost part was occupied by the Iazyges, and the Urgi took up the northern position while the Royal Sarmatians were in the center between the two former tribes. The fourth tribe, the Roxolani, lived east of these between the Dnieper and the Don.
Thus it seems uncontroversial that the Sarmatians - who were masters of equestrian arts - ranged from the Danube to the Don.
Note that the source at:
http://ossetians.com/eng/news.php?newsid=412&f=3
cites the River Dnieper as a facilitator of transportation.