The prehistoric henge monument is a bank and ditch structure. There is a large outer stone circle and two separate smaller stone circles inside the bank.
Wandering through the web looking for information on Avebury and the area around I found this excellent personal site http://www.avebury-web.co.uk/index.html which has lots of information and some fine photographs. It's well written and is worth taking a few minutes to browse through it if you have any interest in times past.