Only the biggest of noobs would believe that this is actually the case, but Our Glorious Comrade Leaders tell us that each style of combat has its advantages and disadvantages against other styles. The styles and their merits are as follows:
BEATING THINGS WITH STICKS!Edit
Otherwise known as melee, it involves picking up something sharp/blunt and hitting your opponent with it. What could be more manly? Generally considered to be most effective against Rangers, but used against just about everything because it is easiest and cheapest.
The different attack styles are stab, slash and crush, and each are good at different things. For example, crush is best against plate armor, and good for pwning that noob in full rune.
Melee is known for its consistent high hits, and a good selection of weapons with useful special attacks. Overall KO potential and cheapness make it by far the best combat option.
Strong against ranged and melee attacks, but weak to magic. Don't worry, they'll still splash every time anyway. Doesn't usually offer any offensive bonuses except for small strength bonuses on some of the high-end stuff. Large negative offensive bonuses to range and magic, however.
FLYING POINTY STICKS!Edit
Ranged involves using either a short/long bow or crossbow to shoot arrows/bolts at your target from afar. Great if you are a pansy. Supposed to be best against magic, pretty much only because of the armor. Basically equally good offensively aganst all combat styles.
You can choose from accurate, rapid, or long-range styles, but there's really no reason to use anything but rapid, unless you're "challenged".
Ranged suffers from the same problem as EVERYTHING THAT ISN'T MELEE, namely zero KO power.
This makes for the best possible defense against magic, and decent against melee and range. Has offensive bonuses towards ranged, and makes for good all-around armor because it does not negatively affect your other combat styles.
OMFG SAD FARCAST NOOB!Edit
Magic is by far the best combat style because of its
extreme KO power, cheapness, devastating special attacks pretty colors.
The runes cost a fortune, the max hit is really low, accuracy sucks, and multiple spellbooks make it a pain in the ass to use for different purposes.
On the plus side, there are a handful of monsters on which it is actually pretty useful (metal dragons, Ice Strykewyrms, and Dagannoth Rex), plus it's pretty fun to use Ice Barrage on that noob in full rune.
Basically useless for defending against anything, but provides most of your offensive magic bonuses. Not much good for anything except farcasting.
So in theory, if your opponent is using range, melee them. Magic, range them. Melee, mage them. Too bad this only really works in Dungeoneering. In most other areas of the game, the balance is so off that even the developers joke about it.