Digital Forensics is the collection, preservation, analysis, and presentation of digital evidence; using analytic techniques to identify, collect, examine and preserve information which is electronically stored. Computer, cell phone and digital evidence can be useful in criminal cases, corporate disputes and civil litigation.
More information is retained on digital devices then most people realise. It's also more difficult to completely remove information than one might think. For these reasons (and many more), computer and digital forensic analysis can often find evidence of, or even completely recover, lost, deleted and tampered with information, even if it was intentionally deleted and wiped clean.