You know when Yoda lifted Luke's X-Wing out of the swamp on Dagobah? Yeah, that's Kremm that is. Kremm is basically the magic power in T&T 7th edition that magic-users draw upon to cast all kinds of wacky spells.
Magic in Tunnels and Trolls is charming and above all, easy to use. In AD&D and 3e D&D spell casting can get a little complicated, having to sit down and memorize from a spellbook, use scrolls that crumble after use and having to take into account casting times and if materials have to be used to cast the spell. In T&T it's simple: If you're a wizard you begin with all 1st level spells and you can use them at a cost of Strength or Wizardry depending if you're using 5th or 7th respectively (although someone will have to tell me whether any other editions utilize magic differently). You must have the right IQ and Dex to be able to use certain level spells. 7th edition is a little more complicated, but I'll get into that.
Let's look at the differences between 5th edition magic and 7th edition. First off, as I mentioned before, when you cast a spell in 5th edition it saps your strength, which regenerates after a short time. I can see why magic has been handled this way but I prefer drawing from the WIZ mana pool in 7th edition because I believe the magic should come from a different ethereal place and shouldn't take its toll on your physical body.
In 5th edition you just spend your strength and the spell is cast, similar to OD&D. 7th edition changes this by having you making a saving roll on INT at a level equal to the spell level to see if you actually cast the spell. So if my Level 1 wizard was casting Call Flame to burn up an orc, he would first make a Lvl 1 saving roll on his INT, which is 14, meaning he would need to roll a 6+ for the spell to work. I like this way of handling spells because it makes spell casting feel like a challenge, like you're concentrating and putting effort into it.
However, one thing I'm not too fond of in 7th edition is the Kremm Resistance. This means you can't cast a spell on someone with a higher WIZ score than yours. If you do you get a "bad feeling" allowing you to stop casting and choose another action. This means your puny WIZ 9 ain't going to do squat to the WIZ 18 warlock looming over you. However, the rules state that if you do cast a spell against the target, the target's WIZ score is depleted by the number of points you used to cast the spell. This means that a group of puny wizards could take down the WIZ 18 warlock by spamming him with spells. I guess the Kremm Resistance rule echoes those bad guys in books and movies who can just hold their hands up to deflect fireballs etc. It's quite cool but it does require a handful of magic users to be in a party.
One of the best aspects of T&T magic is being able to cast spells at a lower level for a discounted price. A level 4 wizard casting a level 1 spell gets 3 STR docked off the casting price, one per level the wizard is above the spell. This symbolizes spells getting easier to cast with practice.
There are a couple more rules such as casting spells at a higher level, purchasing spells from the guild and the different magic classes in 7th edition (conjuring, combat, cosmic and metabolic) but I've covered most of it.
Magic in T&T is really fun, whichever edition you're playing. I tend to prefer 7th edition magic rules minus Kremm Resistance because there's more substance to it and it feels like you really have to try to cast a spell. I have yet to come across a system of magic in a game that is as good as this, but I'll keep looking.
Edit: 6th edition uses Arcane instead of WIZ, but essentially the same thing.