1Onion and mirror
An onion is an address inside Tor; a tor watch mirror is an extra onion address for the same service.
2Relay and circuit
Relays are volunteer servers that pass traffic along; a circuit is the chain of relays your connection uses.
3Bridge
A hidden relay that helps you connect where Tor is blocked.
4Checked link
On this site, a torwatch URL that has been matched against its source and dated.