A tweeter is a small driver dedicated to high-frequency reproduction. There are several types all with their own attributes. Generally the technology involved will have a significant bearing on the overall price of the tweeter.
Cone tweeters are the most common and economical. They have proven to be an efficient and cheap option for a reasonable level of sound quality.
Dome or "edge-driven" tweeters are the type found in most home speakers. They have excellent clarity and sound dispersion.
Unlike most tweeter types which move independently of the woofer, a whizzer is a small paper cone fixed to the woofer's center. Both cones move along a common axis reducing the disconnect that can occur between the two speakers from different angles. The result is a very natural sound.
Horn tweeters deliver low-distortion and superb high frequency sound.