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.

Inner tubes are basically rubber balloons that will expand to fill the available space in a tire. As such there's plenty of latitude in tube sizing. You can see this by the fact that single tubes are rated for a size range ie. from 1.9" to 2.125" as was you original. That said, One size fits all works about as well for tubes as it does clothing.

Most bike shops stock 26" mountain tubes in 2 or 3 width ranges, to cover tires from 1-1/4" wide to 2-1/2 or so. Look on the oiutside of your tire for the width, and buy a tube of the appropriate range. If it's on the border of two, sizes I usually go smaller rather than larger.

The slime option is up to you, it might make sense if you've been plagued with lots of small punctures.

The tubes can stretch and will fit multiple size tires. These numbers represent the narrowest and widest tires the tube will work in. What size is your current tire?