From Wikipedia.org:

A synthesizer (spelling var. synthesiser) is an electronic musical instrument designed to produce electronically generated sound, using techniques such as additive, subtractive, FM, physical modelling synthesis, or phase distortion.

source:http://en.wikipedia.org/wiki/Synthesizer

Synthesizers are used quite often in hip hop production. They are used for melody, basslines, as percussive stabs, and for sound synthesis. The use of synthesizers has been popularized largely by Dr. Dre during the G-Funk era. Modern use of synthesizers is rampant by producers such as Cool and Dre, Scott Storch, and Neptunes. Often in low-budget studio environments or environments constrained by space limitations, producers employ VST instruments in place of hardware synthesizers.

source: http://en.wikipedia.org/wiki/Hip_hop_production#Sequencers_and_Samplers