There are 4 main types of computer that are available on the market. These include:
The wired computers have a cable running up from the fork sensor. These are the most basic type of computers and are very popular. These take a bit longer to fit to a bike and the wire can be damaged if the bike is dropped or the cable gets hooked on something. These computers will only use one battery in the head unit.
The wireless computers have a sensor on the fork that will have a battery. These are quicker to fit to a bike however the batteries in the sensor will need to be changed occasionally - usually changed at the same time as the head unit.
These computers will have a few more functions such as average speed and calories burned.
The 3rd type are becoming the most popular as they have the most functions. These are the GPS computers. You can get speed sensors for your bike if you are using it indoors on a trainer, however if you are riding the bike outdoors all the time this is not necessary as they use GPS to determine your speed.
These will have functions such as altitude and mapping as well. Other optional sensors may include: power meter, cadence sensor, heart rate strap and Shimano DI2 connectivity.
The GPS computers will connect to these sensors using Bluetooth or ANT+ connectivity. You can connect these computers to your phone or computer to upload your rides to Strava or similar systems.
Finally the E-bike computers: These are normally wired into the bike and will give you information specific to that bike only. These computers can tell you your speed, distance and the range remaining in the bikes battery.
For setting up a wired or wireless computer, you will need to know the wheel circumference on setup. Without this number, the computer will give you inaccurate readings.
Below are some popular wheel sizes. The number in the right column is the number you will need to enter into your computer. Some computers will only use the first 3 digits so you may need to round up or down if this is the case.