What would you like to know? If we get enough terms here then perhaps this thread could be made "sticky" so people could refer to it, but to get started:
Sensitivity, refering to speakers, is a measure of how much volume the speaker will produce for a given input power, a more sensitive speaker will make more noise at the same volume setting than a speaker of lesser sensitivity. Typical values are around 88-89db/W(att); more than that and the speaker is very sensitive, less than that, not so much.
A more sensitive speaker will run more happily from a lower power amplifier, so it's a useful figure to know for system matching purposes but doesn't bear any relevance to what the speaker actually sounds like.
Brightness: Presumably you're talking about in audio terms, rather than brightness on a TV, this typically refers to a speaker or amplifier that emphasises higher (treble) frequencies more than another might. Overly "bright" systems can lead to harshness and listener fatigue (hurty ears!).