1. Can anybody tell me that What is the purpose of having 2 NIC(Network interface card) in one m/c (Computer).
2. if I connect two networks by having two NIC cards, then does this harm my two separately protected networks in terms of security (viruses going from one to another nw)

If possible please share architecture of having 2NIC or some reference links, it would be great help for me.

1. Usually, it's to communicate with two separate networks. That's the only way I've used them.

2. Yes, it can, unless you take steps to prevent it.

Addendum to Perito's post:

If you do not have routing enabled on your two nics, then at least the two networks remain somewhat isolated from each other. However, they are still vulnerable to one another based on the security and protection of the computer with both network connections.