Hi,
2n5551 is an NPN bipolar junction transistor (BJT). It is used in those projects where current controlling is required. In 2n5551 small current on base is used to control the large currents on emitter & collector. If you are working on 2n5551 then you should have a look at this tutorial Introduction to 2n5551. They have given detailed description of this BJT. I hope you will get some knowledge.
Thanks.