A seed is sort-of like a BitTorrent seed, or a Bitcoin supernode. It's a public server designed to serve blocks, and connect other nodes to the network.
A witness is a key part of Bitshares-based systems, i.e. Steem, they do many things including generate blocks (yes miners don't mean much on STEEM), handle the valuation of STEEM dollars by producing a price feed, decide on Account Creation Fees, among many other important things.
An RPC server is similar to having a public Bitcoin server, which would normally be an absolutely terrible idea, except with Steem, the wallet and server are 100% separated, so it's completely fine to run a public one. They allow people to interact with the STEEM network from their applications, WITHOUT installing a STEEM daemon locally, which uses a lot of resources.
Thanks for your reply; I appreciate the information.