They are strands of DNA (or sometimes RNA) surrounded in the great majority of cases by protein shells of varying complexity. They have no cellular metabolism of their own and reproduce by entering into cells and by making use of the cell's machinery (such as protein synthesizing ribosomes and in many cases the necessary enzymes).
A widely used way to classify viruses is called the 'Baltimore classification', named for biologist David Baltimore who thought it up. It classifies viruses into seven types, depending on whether they have DNA or RNA, whether it's single or double stranded and other details of the molecular biology of how they reproduce inside cells. (There's lots of variation and it can get very elaborate.)
https://en.wikipedia.org/wiki/Baltimore_classification
Viruses also differ a lot in their protein coats. Some are just featureless spheres, but others have lots of parts. (The virus' shell is important, since viruses have to make their ways through cell membranes. So the proteins on their coats will often bind with receptor sites on cells, stimulating them to engulf and ingest the virus. Others inject their DNA like hypodermic syringes.
https://en.wikipedia.org/wiki/Bacteriophage
The usual answer is 'no'. That's because they don't have any metabolism and they can't reproduce by themselves. They manage reproduction by hijacking living cells.
Either DNA or RNA.
Where did they come from?
Nobody knows. Considering that they need cells to reproduce, the most widespread idea is that they arose after cells did.
But their (relative) simplicity does make the idea attractive that they are descended from the kind of pre-living chemical replicators that hypothetically also produced the first cells like LUCA (last universal common ancestor). Except that viruses would have subsequently lost the ability to reproduce on their own, perhaps because it was more efficient for them to use the cellular machinery of the cells around them.
https://en.wikipedia.org/wiki/Viral_evolution