With so many options out there these days to choose from, deciding which type of hidden camera to buy can be challenging to a novice. The quality of picture you will get depends on the sensor in the camera and the range of the transmitter, if you are using a wireless camera from a distance or though walls. Furthermore, if you are planning to use a hidden camera in the dark or pitch black, you will probably want to consider IR (infra red) cameras.
The sensor types commonly available are CMOS, standard 420 line CCD, and improved CCD (450 lines and up)
The number of lines doesn't really matter that much--what really matters is the clarity of the picture. A CMOS camera has 380 lines, which sounds pretty good. In reality, it's nowhere near the quality of a 420 line CCD because the picture is noisy and blurry. The advantages of CMOS are the low cost, small size, and low power usage, which makes it possible to power them with a standard 9V battery.
CCD is so superior to CMOS, most users will never go back to CMOS once they have seen it. The advantages of CCD are a sharper picture, good performance in low light, and better manufacturing (they tend to last longer).
Improved CCD sensors are on the market, though much more expensive and harder to find in a hidden camera. Usually, you will find them in standard security cameras instead, which commonly offer 500-550 lines of resolution. Pinhole CCD sensors are more likely to be 450, which is plenty for most applications. The advantages over standard CCD sensors are higher quality and smaller size. Light sensitivity tends to be about the same.