The violin is the highest pitched instrument in the string section. It has a resonant lower register and can also reach extremely high notes. As with many orchestral instruments there is no definite top note - it all depends on the skill of the player.
There are four strings, usually tuned to G3, D4, A4, E5. Each of the strings have different characteristics which composers exploit when writing music. For example a melody played high up on the G string will have a darker, thicker sound than the same melody at exactly the same pitch played low down on the A string.