I'm exploring systems to play after 20-25 years out of RPGs. Alot of the mechanics of BW appeal (I think) after reading through the downloads available, buit I'm a little fuzzy on how magic works. I assume doing something magic is a test, in some sense, no different than swinging a sword or bashing down a door. But specifically, things like spell lists (which I don't like), fatigue, is magic broken into areas, is is open ended to allow for scaling of effect, etc etc?
Sorcery in BW is a skill. However, trained sorcerers get to roll both their Sorcery skill and their Will stat to cast spells. These tests are open ended, to the base obstacle for the spell (though only some of the spells increase in effect with extra successes). Casting spells has a price - a physical tax on the caster's body (the player has to make a Forte test after casting a spell, at the same obstacle as the spell). There are other effects for failed spell casting.
There are spell lists in the character burner, but Luke has developed a guide, available on the wiki (http://www.burningwheel.org/wiki/index.php?title=Magic_Burner_FAQ),to help you build your own spells.
You can check out some of the spells players have created at the BW forums (http://www.burningwheel.org/forum/index.php).
Hope that helps.
To add to what Mayuran said:
There are spell lists, but it's not a fire-and-forget spell system. If you know a spell, you can cast it. However, there is a price. Everytime you cast a spell, you must test your Forte stat against the difficulty of the spell. Failure on this roll taxes you. You can potentially cast yourself into unconciousness or even death unless you are careful.
There are other options as well. We have two free chapters from a planned book called the Magic Burner, available on our Wiki here (http://www.burningwheel.org/wiki/index.php?title=Downloads&burning_bwwikidb_session=aaa3b434c87b9220f592dac0684812a0#Magic_Burner_Sample_Chapters). The Abstractions chapter allows you to build spells from their components (facets) on the fly. It is very dangerous, however, and the recommended use is to individual facets to alter spells you already have. For instance, the Eldritch Shield spell, which protects you against hostile magic, has the Caster facet. That means that it can only affect you, the caster. However, if you know the Single Target facet, you could alter the spell slightly on an ad hoc basis to cast it upon someone else.
The other system is Summoning. The document that is up there now will be revised at some point in the future (it was originally written for an older version of the rules) but is close enough for your purposes. It is a more freeform magic system.