I've seen three different types. The first is just you basic ram, carried by troops and manually swung against the castle gates. this was somewhat effective, but costly in manpower at the only protection they had agains the defenders was shield held over there heads. when one troop fell, someone would have to rush to take his place.
The second type is a long log mounted on wheels. the troops would pull it back aways then rush it forward to connect with the castle gates. this one is more powerful and has the advantage that a covering could be built over it for protection. the downfall to this one is that depending on the approach to the castle gates, you might not be able to get alot of momentum.
The third type is also the most powerful. it is a log hung between two a frames as a swinging ram. this one has the advantage that you could get alot of hits in rather quickly, did not take alot of troops to operate once in place, and could have a cover built over it. the problems with it is that it was rather difficult to get into place.

what was used depended largely on the approach to the castle gates, and the defenses present. you can go online and there are some good sites describing these weapons. this is one of my favorites.

http://www.medieval-spell.com/Medieval-Sie...ge-Weapons.html

HYDRO