Modern string players tend to avoid playing the open strings of their instruments where practical by using different fingerings. However, open strings create a more resonant sound and composers quite often exploit this in their writing. The open strings of the violin are G, D, A and E, whilst the viola and cello are C, G, D and A.
In the Haydn example below, the G string on the first violin is the lowest note so must be played open as are the bottom Cs on the cello. All the drones in the Shostakovich example are also the lowest strings. As well as being more resonant, open strings tend to sound a bit plainer as vibrato cannot be added:
Haydn Quartet Op. 9 No. 1, first movement
Shostakovich String Quartet No. 8, first movement
An more unusual technique involving open strings is bariolage where an open string is alternated with the same note stopped on a lower string, creating a wah-wah effect that was used by Haydn as well as Brahms in the example below:
Brahms Quartet op. 51/1, second movement, trio
Vibrato is a slight by steady fluctuation in pitch which is added by pivoting on the relevant finger of the left hand, causing the note to bend above and below the main note. String players will usually make the decision themselves how much vibrato to add to a given note but some composers have exploited the contrast between vibrato and ‘non vibrato’ or ‘senza vibrato’ as in this example from Bartok. Listen to the relative coldness of the non vibrato at the beginning compared the warmer vibrato chord at the end of the bar 3.
Bartok String Quartet No. 4, third movement