One major detail that has kept me from multiclassing has been the question of whether multiclassing Base attack Bonus counts for getting additional attack as part of a full-attack action.
Spellcasting DC is more clear as part of your spell DC is your caster level. I do wonder if have multiple caster classes created different "caster level Pools" or just all classes with the ability to cast stack.
One major detail that has kept me from multiclassing has been the question of whether multiclassing Base attack Bonus counts for getting additional attack as part of a full-attack action.
Spellcasting DC is more clear as part of your spell DC is your caster level. I do wonder if have multiple caster classes created different "caster level Pools" or just all classes with the ability to cast stack.
You add up your primary base attack bonus (The first number) then see if you get multiple attacks from that. A barbarian 3/fighter 3 would have a primary BAB of +6, and so would get a second attack at +1, the same as a Fighter 6 or Barbarian 6.
Spell DC has nothing to do with caster level. It is based on ability scores and spell level. Still, all caster levels are separate unless they specifically say they stack (And as far as I am aware, only prestige classes have that language).
BAB stacks between classes to determine when you get iterative attacks. You always get the first iterative when your BAB hits +6, another at +11, and the final at +16.
Caster level for classes is kept separate. If you have five levels in wizard and two levels in cleric, then wizard spells you cast work with a caster level of 5, while cleric spells you cast work with a caster level of 2. Similarly, those two cleric levels do not count when it's considering whether spells you cast as a wizard get a heightened benefit, such as additional projectiles from Magic Missile or Scorching Ray.
So multiclassing warrior classes is alright, but spellcasters only works if they are prestige classes like the Mystic Theurge?
Correct. Note that there are some ways, such as the Magical Knack trait, that can increase your caster level in a class in a way to mitigate the drawbacks of multiclassing somewhat. (Though it affects only caster level, not any other perks for leveling a caster such as more spell slots or spells known, that prestige classes might increase.)
Ive read up on some of the prestige classes with spells to understand how they work.
The description is odd as it says it is effectively an additional level in the class for everything(I know of) related to spells.
You gain more slots, more spells known(unless you use a spellbook), and higher caster level for all effects like duration and DC.
So is there something missing? if not then putting some levels in a magical prestige class and using the Magical Knack trait could give a large bonus, especially if you dont like your original classes features.
Back in the days of D&D 3.x, most classes didn't have very many features other than spells (when applicable), BAB, and Saves. Prestige classes often give you more class features while progressing spells at anywhere from 3/4 to full speed (depending upon the class feature mix), so if you were a Cleric, Sorcerer, or Wizard, which were REALLY bare on class features (even more than today), you would usually want to get into a prestige class as soon as possible.
In Pathfinder, classes generally have more class features; Cleric is an exception -- all you get is more Channel Energy, unless you pick an archetype that trades out part of this. Wizards probably get the next least after Clerics, but even their Arcane School abilities often give something of interest, although Arcane School quality is quite uneven, and even among the good ones, certain Arcane School abilities are very nice without the requirement for scaling (example: Admixture sub-school of Evocation) while others really need their scaling (example: Teleportation sub-school of Conjuration). Sorcerers get cool stuff with their Bloodlines, but these are really uneven in quality, so some (like Arcane) are awesome and you don't want to prestige out; some (like Abyssal) give you really good stuff after a delay (the 9th level Bloodline Power is the one you really want, and it is heavily dependent upon continuing to get Sorcerer levels), while some give you something really good at the start (like Ghoul -- the awesome 1st level ability does get an upgrade at 7th level, but then no more), while some are kind of bad (Destined and the Envenomed Wildblooded variant of Serpentine). So if you are a Cleric that isn't getting much use of Channel Energy, a Wizard with an Arcane School that doesn't depend much upon level, or a Sorcerer with a Bloodline that doesn't scale well or whose good stuff starts way late, going into a prestige class can still be a good option. On the other hand, if you are an Arcanist, Druid, Psychic, Witch, or ANY 6/9 or 4/9 spellcaster, you depend heavily upon class features other than spells, so you don't want to go into a prestige class unless you can get one that progresses class abilities other than your spellcasting. Oracle is sort of in between -- Oracles get Revelations as they increase in level, but some of these scale, and some of them don't, so going into a prestige class may or may not be detrimental.
Notable exceptions from the base class end: Pre-Errata Orc Scarred Witch Doctor could be an awesome enough Gish that you could take a martial dip and go into Eldritch Knight and do well enough to make up for the lack of Hex progression . . . And then the Scarred Witch Doctor Errata happened. (Sniff . . . .)
Notable exceptions on the prestige class end: Evangelist (not to be confused with the Cleric archetype) levels other than 1st progress EVERYTHING from your base class except for having the prestige class' BAB and Save progression, so if you can hack the steep daily requirement of Deific Obedience, go for it, even if your base class otherwise is highly unfavorable to prestige classing. Short of this, a few prestige classes (such as Dragon Disciple) have their own specific progression of certain base class features, and one really rare example (Inheritor's Crusader) even works about right with both Cleric and Paladin.