Not quite, a bass synth Takes the signal from your bass and manipulates it, rather than a straight electric wave signal. There are lots of ways to make a bass [i]sound[/i] like this, without actually doing it though!
Edit, But you can play a Synthesizer in the bass registers also.
You can more or less make any sound you want with one. Typically renowned for that warm fuzzy bass - Lots of soaring Leads and Epic Pads are all synths or variations there of.