That's not simple?

er....

Putty is a program you use to get a shell on a remote computer through one of the protocols mentioned above.

er...

Putty is a program you use to get a commandline that is running on a machine far away (or in the same room even) to appear on your own machine like you were really sitting at it.

er...

I don't know how to say it any plainer. Hope that helped.

Infinite