Are you looking to create one? The best start is by reading H.323, H.225, and H.245. Since you will probably want a GKRCS model I placed in H.245. By using this model you will be able to create creative applications on top.
SIP seems to have more hype than H.323 lately especially since H.323 is considered one of the older technologies and if you create something now you have really lost out on the Time to Market value of a product.
SIP also is probably easier to develop as it is all IETF drafts. You may want to look into this type of implementation. There are some freeware stacks on http://www.vovida.org.