when to use merge,transactional replication

+1 to get this off the unansered posts list.
Please refer to BOL for general information and then please come back with more specific questions.

SQLdbarocks (a screen name to apt about DBAs), But I see you have lots of questions asking on basic information. As you might have already got the response referring to Books Online which si a first hand resource for all your doubts. Not that we don't want to respond to basic questions.
If you can take your time by searching about TUTORIAL in the forums, you will get related threads talking tons of information in this regard.
Hope this helps.