Basically, JWs believe that there are two classes of Christians. There are the annointed remnant, and the "other sheep". The annointed remnant are made up of 144,000 born-again Christians.
Jehovah's Witnesses believe that they are the only true Christians on earth. They believe that most Christians throughout history (post Apostolic period) were not true Christians; therefore were not part of that annointed remnant.
This doctrine is very closely tied in with the Faithful and Discreet Slave doctrine. The Faithful Slave are the 144,000 annointed remnant; begining at Pentacost. JWs believe that there were always some members of the Faithful Slave throughout history; and that C.T. Russell was one of them.
JWs believe that these "Biblical truths" were always present within the Scriptures, but it took C.T. Russell to revive these teachings, and for the organization of Jehovah's Witnesses to make these "truths" known to mankind.
There's so much more that can (and should) be said about these doctrines, but that is all I have the time for right now. Hope that clears some things up for you.