The violin is a bowed string instrument. It usually has 4 strings and is an important instrument in both classical music and traditional music in different cultures around the world.
Also known as fiddle in folk music context, the violin is also one of the fundamental instruments of classical musi. A typical symphonic orchestra has between 20 and 30 violin players usually split into two groups (Violin I and Violin II).
The violin is tuned in 5ths, G > D > A > E, being G the lowest. Other tunings can also be found.
For more detailed information, you can refer to this Wikipedia article