How do I reassemble packets from a packet sniffer?

I have been trying to understand information from Wireshark (as a hobby on my own network!) and was wondering how I can reassemble the packets into something I can understand.  Is there another program that does this?  (Preferably freeware or trial-version but I'm not against buying it, depending on the cost)  I would eventually like to be able to sort out different types of traffic (e.g. instant messaging, web site traffic, e-mail, etc.) and understand what the user is looking at.  I am a definite newbie with Wireshark and any help is greatly appreciated.  Again, this is strictly for my own use on my own network so I'm not doing anything nefarious!!!  :)