Ceiling speakers are a great way of integrating speakers discretely within a room. They can be used in single pairs or linked together in multiple pairs to evenly cover larger areas with sound. The diameter of the bass driver is a common way to categorise a ceiling speaker, in a similar way that TV's are measured by the diagonal size of the screen in inches.
Most ceiling speakers point directly downwards and have a swivelling tweeter to help aim the higher more directional frequencies to the general listening area. For installations where the sound needs to be "pointed" towards the listening position, such as in a home theatre setup, models with a swivelling bass/mid driver and tweeters are recommended.
The low frequency response (amount of bass) of a ceiling speaker always tends to be much less compared with the equivalent book shelf or floor standing speaker. This is because the speaker does not sit within a specific tuned ported cabinet but instead has to operate in "free air".
A way to achieve a large full sound with a solid bass response is to use a separate subwoofer within the room. As the subwoofer produces only the lower end of the bass spectrum, which is unidirectional to the human ear, the bass will appear to come from the ceiling speakers. The subwoofer can be placed discreetly within the room, behind a sofa, under a side table or even integrated into custom made furniture.