A seed node is simply any Steem / Hive server which has it's P2P port (usually 2001) open to the public, and the seed URL posted publicly.
Witness servers generally have their public seed port firewalled off, for extra security - preventing an attack against the open seed port. This generally means they can only connect to servers which have their seed port open - i.e. actual seeds.
A seed node simply relays transactions / blocks between other seed nodes, witnesses, and RPC nodes etc.
Some people run a "light RPC node" (@aggroed calls them "compact broadcast nodes") on their seed node's, which provide a small subset of the features that a "full RPC node" provides - usually enough to handle something like Hive Keychain, but not applications that require post data (like peakd.com / hive.blog), or account history (l ike https://pay.privex.io and https://beeanon.com )
Seed nodes aren't linked to an account, but the person who runs it is usually clear from the domain it's attached to.
They don't receive any rewards, but they do help show goodwill to the community / network - which is important if you're a witness, since witnesses are similar to politicians, trying to convince people to vote for their witness.