Ok, it can be a little confusing to begin with I'll admit, but once you get your head around it it's not too bad.
Level Adjustments work in conjunction with HD.
For instance, if you were to play a Doppleganger (4HD and Adj +4) you would start with a lvl 8 monster with no class levels.
This does generally mean that you have a character who is weaker in their class levels, but they have more bonuses. An example of this is a lvl 10 Barbarian has 10 D12 HD and all abilities for lvl 10.
A minotaur of the same ECL (Effective Character Level) however, would be a lvl 2 Barbarian (6HD, +2 Adj and 2 levels in Barbarian).
The downsides here, is you only have 2 levels in Barbarian for abilities and also only 2D12 HD. Then 6D8 HD from the minotaur levels.
Which also reduces BAB, Feats, Ability increases and obviously the number of HP from the Con bonus.
However, they gain more than they lose as they are large to begin with and have +8 Str, +4 Con, -4 Int and -2 Cha (obviously a combat character lol) and natural armour +5, Dark Vision 60ft, Natural Weapon (Gore), Special Attack (Powerful Charge) and Special Qualities (Natural Cunning and Scent).
So for all bonuses there are penalties.
If you have creatures like a Drow which has no racial HD, then it's just Class Levels + Level Adj.
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A word of warning. Be careful when allowing monstrous creatures for characters.
Imagine what a village would do if a Drow or a Minotaur/Troll etc walked through the gates? There would be panic, racism at the very least (my Gnoll once gained entry to towns when he had proven he was not evil, but he was continually challenged and rebuked if he fought back. Things cost 50% more etc) hostilities are very likely also.
If a group of drunks see a lone Drow they are likely to grow hostile and likely attack.
These are things to be wary of.
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Edit: If you want a character similar to a hybrid without shapechange etc (my group never allows curse of lycanthropy btw, even if you start as a werewolf as it can cause player issues). Just roll up a gnoll.