How do I know which package is which?

[Soo-Hyun Choi]

For example, if I would like to install GNU Plot, how do I know to
which I need to type when I do "sudo apt-get install <what-to-say>"?

Let say, how do I see all of the package lists which can be installed
by 'apt-get'? I've looked up Synaptic but seems it doesn't say
everything.

Thank you.
Soo-Hyun