I'm sure you've seen this:
http://www.wowwiki.com/COMBAT_LOG_EVENT_Details
Which is pretty un-user friendly, but has a lot of info. here are some quick tips:
1. The only event you care about or want to register is "COMBAT_LOG_EVENT_UNFILTERED"
2. The second argument of the above event is also called "event" but it is not an event, it's a string representing the sort of thing that happened to trigger "COMBAT_LOG_EVENT_UNFILTERED"
3. There are 8 base arguments from "COMBAT_LOG_EVENT_UNFILTERED", they are: timestamp, event, sourceGUID, sourceName, sourceFlags, destGUID, destName, destFlags
4. If you just want to get the same functionality as the 2.3 log, all you care about are "event", and the two "name"s
5. For the most part you can ignore "flags," if you aren't used to them. They're there to help filter things faster, but you can still use "name" to do that yourself.
6. GUIDs are useful if you need to seperate entities that have the same name, for example:
if destGUID == UnitGUID("target") then --the thing that got hit *is* my target, and I know this even if there are other things with the same name
7. Each possible value of "event" (arg2) has its own set of additional arguments that get passed *after* arg8 (destFlags)
7-a. Up to 3 of those are determined by the first word of "event", for example "COMBAT_LOG_EVENT_UNFILTERED" events which have an arg2 that starts with "SPELL_", have an arg9 "spellId"
7-b. Up to 8 more arguments *after* the 3 from the last point are determined by the remaining words of "event", for example if arg2 contains "_DAMAGE" then the event has an arg12 of "amount"
Hopefully that helps in decoding the wowwiki stuff somewhat better.