A dinghy is a small, open boat, often propelled solely with oars or a scull. NOwadays, the term often applies to a smaller boat of the above description carried by a larger for, in order to access areas where the larger boat would be at risk (e.g. accessing a sloping beach where you'd dangerously ground a larger vessel, but can row a dinghy right up on the beach).
According to thefreedictionary.com a dinghy is:

1. A small open boat carried as a tender, lifeboat, or pleasure craft on a larger boat.

2. A small rowboat.

3. An inflatable rubber life raft.