Deleting..?? (deleting and reinstalling an app)

Hi,

If i delete certain applications from my macbook (Garageband for example), i always have the option of installing it again if i need it from the install discs, Right?

Ok, when it comes to installing it, can u just pick out the application from the disc and just install that single application? or do u have to install the whole MAC OS X.

Im asking cause i dont see an option for individual applications on the install disc.. just MAC OS X as a whole..

Im new to macs so i guess u can understand.....

thanks, P......