Cycle Computers vary widely in function, style and price. Below is a rough guide:
These tend to be wired with basic speed, average speed, distance and time functions.
Usually wireless with no wire between the sensor and bar unit. These have the same basic functions as the entry level but may have other additional functions including stop watch, split time etc and may offer compatibility with other training aids including cadence sensors, HR monitors or other such devices.
At the high end you will find GPS enabled computers that do not require a wheel sensor. These units offer all the basic functions and are usually equipped with larger led screens (mono or colour - many are touch-screen). They have the ability to store maps; that can offer satellite navigation, and the facility for storing training data that can be uploaded to your home computer or laptop. Many are USB rechargeable with memory cards, and offer good battery life. With the correct software some units can be paired with your mobile phone for accessing texts and messages. They are mounted easily to bars or stem with band or bracket fixings.