ssh agent-forwarding
Although it culminates at agent-forwarding, this document is a nice intro to ssh at the non-technical key exchange (kex) level (this means, it talks about private and public keys but without maths or CS theory). It helped me a lot and takes only a few minutes to read. Recommended for anyone using ssh (and especially if you're still using passwords!).
http://www.unixwiz.net/techtips/ssh-agent-forwarding.html
http://www.unixwiz.net/techtips/ssh-agent-forwarding.html