Guests use the two words interchangeably when they enquire, and then arrive expecting the other thing. The difference is worth five minutes of your time.
The felucca
A felucca is an open wooden boat with a single lateen sail and no engine. There are no cabins. It is the right choice for an afternoon between Aswan and the botanical island, or a sunset that you will remember longer than the photographs of it. It is the wrong choice for a week, unless you are twenty-two and sleeping on deck sounds like the point rather than the compromise.
The dahabiya
A dahabiya carries two sails and, usually, between six and twelve cabins. It sails when the wind serves and takes a tow when it does not, which is why it can keep an itinerary. Because it is small, it can moor at banks the large ships cannot reach — you wake up against a sandbank with no other vessel in sight, rather than rafted three-deep at a town quay.
If your week is about the river rather than the mileage, this is the one.