| An example of a hammer-on would be to fret a note with, say, your index finger. Pluck the note, and then, while holding this note with your index finger, bring down your middle, ring, or pinky finger onto another fret on the same string. If you do it with enough force (it doesn't take much), the fret that you brought your finger down on will sound without having to pluck it. You can also hammer-off, or pull-off. An example would be to have two fingers pressed down on different notes on the same string-pluck the string, then take whichever finger is closer to the bridge off, and the note that your other finger is holding down will sound. You can do these with any arrangement of fingers, you can do it across several strings, etc. It's basically just fretting a note hard enough where it sounds without having to pluck a string.
The "go up and down" thing you're talking about is called a slide, and you don't need a fretless to do it. "Come Together" was not played with a fretless, IIRC. It just takes a little practice to get a slide to sound smooth without all the fret noise-watch how much pressure you apply with your sliding fingers, and don't EQ your bass or amp too trebley if you don't want the fret noise. | 
