Double bass is an instrument of bowed string because we need a bow. The strings can be also played with fingers, doing pizzicatos. It has four strings and it is the biggest member of the string family, with the longest strings, which allow it to play very low notes. You play the double bass standing up or sitting on a very tall stool. In the orchestra double bass is usually located behind the other string instruments and to the right of the conductor.
In this picture, several parts of the instrument can be seen:
Musical Instruments 