Roman criminal law was a little - or rather a lot - harsher than criminal law today, at least in most western societies. When one thinks of punishment in Roman times, images of criminals being crucificed or eaten alive by lions (damnatio ad bestias or Latin for "damnation to beasts") in the coliseum directly come to mind.
Roman punishment actually varied depending on one's position in Roman society. A slave had no rights whatsoever and was literally treated as merchandise. A slave would commonly be beaten for various offenses.
Another form of Roman punishment was to mark a slave's forehead. A slave could also be forced to carry a piece of wood round his neck. This humiliating form of punishment was called the furca. A slave caught committing theft, adultery or forgery could be punished with death (sometimes by crucifixion).
It is worth noting that being a merchandise, the slave also had a cost and therefore the corporal punishment could never be too harsh in order not to permanently impair him. Often slaves would be sent to the equivalent of a labor camp today to perform tasks such as turning a mill or grinding corn.
There were different kinds of punishment, the most common being fines (damnum). The worst forms of punishment were infamy (ignominia) for theft, interdiction from fire and water, banishment from Roman society (exilium) which meant that a Roman lost all his privileges and property or even worse: slavery (servitus) or death, but only for treason or patricide. Banishment could occur for offenses such as forgery or false testimony.
For theft the common punishment for a Roman citizen was to pay damages usually many times the value of the object stolen. The Romans made the difference between manifest and non-manifest theft, which depended on on how close the thief was to the scene of the crime, manifest theft being the worst kind of theft. Initially, the penalty for manifest theft could be flogging, slavery or even death. Later on it was changed to paying damages amounting many times (usually four times) the value of the object stolen.
The punishment for patricide was to be thrown into a river. Before being thrown into the river, the criminal would be blindfolded, stripped naked, whipped, bonded and then put in a bag (sometimes a serpent, a dog or a cock would also be placed in the bag). Other ways of inflicting death included: beheading, strangling, throwing a criminal off a high building / cliff, burying a criminal alive, crucifixion (reserved for slaves).