The word 'domain' means an area or a place. So in a network a collision domain is the area within the network in which any collisions that occur will be propogated. The effect of those collisions will be felt everywhere within the collision domain, but outside it (or in a different collision domain) their effects won't be noticed. For example, a hub is a shared-access device so collisions on one port will affect traffic using other ports; the whole hub is one collision domain. A switch however does not propogate collisions between ports so each port on a switch forms its own little collision domain - the collisions affect that port only.
Broadcast domains follow exactly the same logic except we are talking about broadcasts and not collisions. A broadcast domain is the area within the network within which broadcasts will be propogated. Broadcasts are passed on by switches but not passed on by routers so adding routers or using VLANs is a good way to break up a network into separate broadcast domains or to limit the size of broadcast domains.
I hope that makes sense; it seemed easy to explain until I started trying to explain it...
thanks for clarifying my doubts. i am new to networking. i am new to basics also. i am reading this site and its very useful to me. but i need some guidence. if u have some for me, plz give me the topics that i have to prepair from basics. i mean, which topic i have to read first and then which topic like that. i need the list of topics in order. so that i will concentrate in that manner.
actually i need the list of topics that i have to prepare, for total networking from basic concepts...
That is a tall order! I'd recommend getting hold of a copy of the official Cisco CCNA material. That will give everything that's required from binary through the OSI model, ethernet, routing, switching to WAN technologies etc. etc.