Computer Forensics is often used when investigating crimes which are related or conducted using a form of electronic equipment- and not necessarily just computers. It can include analysis of cellular phone data/images, compact disc, digital video discs, portable storage devices such as flash drives, etc...
Often it involves recovering images or data from a hard drive of a computer or electronic device so that it can be analysed and evidence can be located.
Examples of this would be the seizure of a computer from a pedophile, so that the hard drive can be analyzed and the illegal images of children can be used for court evidence.
Another example would be to locate deleted messages sent over email to establish a link between a victim and suspect.
It can also include analyzing the hard drive to determine if the user may have been involved in hacking or performing some other form of illegal activity over a private network or internet.
It's obvious that an investigator must be extremely skilled and well versed in various types of computers and electronic equipment.
They must also be extremely patient as the amount of data that must be analyzed and sifted through can be overwhelming.
Below are a couple videos which provide more info.