A core build up is used as an additional procedure in conjunction to doing a crown. If the tooth has a large amalgam or composite filling  and there is recurrent decay under it, the old filling has to be removed as well as the decay. This leaves a shell of a tooth and a core build up is placed in the hole to re-enforce the tooth and keep the tooth from breaking. The tooth is then prepped for a crown and the impression is taken to have the crown fabricated. A temporary crown is worn for a couple of weeks till the crown is completed.
A post and core is a procedure which is sometimes done following a root canal.  If the tooth is severly decayed and there is not much tooth left to place a crown on, this procedure is done to add an extension to the tooth for crowning. Some of the root canal filling material is removed and a metal post in place down into the canal. Hard core material is placed around the post and once dried, the tooth is then prepped for a crown.	The tooth above is now ready for the post to be inserted into the canal.
The tooth shows the post completed and is now ready for the crown prep and impression. A temporary crown will be placed while the custom crown is being fabricated.	The end result is a pretty smile and much better chewing ability.