What is the role of viruses in cancer?
Some viruses (retroviruses) insert their genes into the host's DNA in order to force the host to copy their genes for them. If this insertion happens in certain places, it can interfere with the host cell's own replication mechanisms, with the possibility of leading to uncontrolled cell division.
This uncontrolled cell division can lead to tumour growth.
This is a very basic outline - If you would like a more detailed explanation let me know.