Instead of constantly doing the same task (woodcutting, fishing, etc) you have a variety of tasks you can do which involve many different methods. You collect herbs, runes, rare items, and most importantly experience. Say you have a task dealing with a monster that drops herbs and seeds. While killing it, you gain combat (Att, Str, Def, Magic, Range) exp along with HP experience. With the bones that it may drop you gain prayer experience. The seeds can be harvested for farming experience, while the herbs it drops can give herblore experience. Overall Slayer is a good method of overall training that I think everyone should try. I also agree with Campy's statement above. ;)