choked
When a connection is choked, it means that the transmitter isn't currently sending anything else on the link. A BitTorrent Client signals that it's choked to other clients for a number of reasons, the most common is that by default a client will only maintain --max_uploads active simultaneous uploads, the rest will be marked choked. A connection can also be choked for other reasons, for example a peer downloading from a seed will mark his connection as choked since the seed has no need to receive.

snubbed
If the client has not received anything after a certain period (default: 60 seconds), it marks a connection as snubbed, in that the peer on the other end has chosen not to send. See the definition of choked for reasons why an uploader might mark a connection as choked. The real function of keeping track of this variable is to improve download speeds. Occasionally the client will find itself in a state where even though it is connected to many peers, it is choked by all of them. The client uses the snubbed flag in an attempt to prevent this situation. It notes that a peer with whom it would like to trade pieces with has not sent anything in a while, and rather than leaving it up to the optimistic choking to eventually select that peer, it instead reserves one of its upload slots for sending to that peer.