I'll make it simple in a way that will ultimately complicate it even more and lead to further confusion.
Combat level is your overall level that is shown on your combat menu. Other levels are shown on your level menu and include things like attack (how good you are with a sword), range (bow and arrows), woodcutting, firemaking, runecrafting, magic and various others.
My highest stat is range, at 35, which means I am high enough a level to use a maple longbow (highest normal bow) and mithril arrows (highest normal arrows) although I just use bronze (lowest normal) because they are cheap and I have over 2000 of them. :3
Right here, right after the first sentence of the paragraph beolow did I get confused (After the defence=take more/less damage part).
My lowest stat, however, is defence which means I will take more damage than I should, but seeing as I specialise in range, the chance of taking damage from physical attack is reduced by distance and time it takes for the foe to reach me, and magic is weakened by the particular defensive clothing I wear.
Now, are you confused yet? o3o
*is shot thrice more*
*dies*
*waiting for lag to reduce before respawning*