The Roman empire is the easiest for newcomers to Travian. Because of their significant social and technological developments, the Romans are the masters of building coordination, and their troops are the Travian elite.
They are relatively decent in both attack and defense. To get this versatility, however, they must go through a long and expensive training. Their infantry is legendary, but their defense against cavalry is the lowest of all the tribes.
For beginners and those who don't know exactly how they want to play, the Romans are ideal.

The Gauls are the most peace-loving of all the tribes. Their units are well-trained in defensive tactics, but their offense is lacking in comparison to the other tribes. Gauls are born riders, and their horses are legendary for their speed, allowing them to move quickly and surprise their foes.

This tribe is relatively easy to defend, but an offensive play style is also quite possible. It offers the possibility to go in every strategic direction (offensive or defensive doctrine, lone wolf or helper in emergencies, infantry- or cavalry-based, settler or conqueror), so anything is doable for a skilled player, but also good for beginners!

The Teutons are the most offensive of all the tribes. Teutonic warriors are dreaded everywhere because of their berserker rage in battle. They are the pillaging hordes roaming the lands, completely unafraid of death.

Because the Teutons lack the military discipline of the Gauls and Romans, they are somewhat slower and weaker than other troops. For offense-minded and experienced players, the Teutons are a great choice!