I ride a mountain bike, and am starting to do a little trail riding. I just blew my back tire, and need to get a new tube, and I don't know what to get. The old tube is made by Kenda, and the size is 26 X 1.9/2.125. I looked at wal-mart and there were two kinds. One made by a company called slime, that's supposed to reseal itself if punctured. The closest I can find to the size of the old tube 1.5/2.125. The other kind was like half the price, and said that it was a universal tube, made by Bell I think. I didn't even give a size except that it was for a 26 inch tire. What do the numbers 1.9/2.125 on my old tire mean? Are these slime tubes any good? Are the Bell universal ones any good? What are the best tubes to get? Are the front and rear tubes generally different sizes?

I'm a complete amateur when it comes to this stuff. any help would be appreciated.