So, I live in NY, and we have stricter (but very, very similar laws) to CA. So, I'll try to shed light on the topic as best I can:
In states like CA and NY, there's basically 2 "classes" of civilian weapons: "Assault Weapons," and "Non-Assault Weapons." An assault weapon is any semi-automatic gun that has a detachable magazine and at least one "feature." The banned/illegal features are as follows:
A pistol grip (a grip which protrudes beneath the weapon, at an angle of 45 degrees or greater), a thumb-hole stock, or "bump-fire" stock;
A threaded muzzle;
Any type of muzzle device, including suppressors, compensators, muzzle brakes, flash hiders, etc;
Any type of bayonet mount (or a welded bayonet - but who the hell does that?);
A grenade launcher (or mounted flare gun - DD [destructive devices] are already banned in CA and NY, so this would only apply to flare launchers);
A "second hand grip," such as a vertical grip. Magwell grips, fore-end magazine-holders, certain types of bipods, and certain types of angled foregrips also count as "second hand grip." Standard fore-end covers for most guns do not count as a "second hand grip;"
A folding, telescoping, or detachable stock (the "detachable" portion is not listed by law, but since a rifle is defined as being "shot from the shoulder," the absence of a stock [or ability to remove one] changes the guns definition and thus no longer counts as a rifle. Legal issues ensue, so they just banned it entirely);
A barrel shroud.
NY has a few extra, really dumb clauses:
Any magazine that is legally owned (e.g. 10 rounds or less) may not be loaded with more than 7 rounds of ammunition, at any time;
Barrel porting is banned as well (since it runs afoul against the newer laws passed in 2013);
Bullet buttons, or similar locking mechanisms, are explicitly banned. Magazines must either be welded or riveted to their host weapon, and the magazine locking hardware must either be welded over or riveted into place;
Fixed stocks that have adjustable lengths-of-pull / rise count as "telescoping," and are banned;
Rifles where the action folds in half (such as the KelTec SU-16) are considered "folding stocks," and are banned (even though the gun is totally inoperable once folded);
Various models and "all copies and variations therein" of weapons are banned: "Kalashnikovs" and "AR-15 (all models)" are both explicitly banned. A lot of companies get around this by selling their weapons with different model numbers, but there has been cases (at least in NY) that establish the precedent that "all models" covers these variations as well - effectively making everyone who owns an AR or AK in these states a felon. Other guns are banned by name, but they're specific models made by specific manufacturers.
THAT ALL BEING SAID, if your gun has a fixed magazine, of the legal capacity or less, then you're allowed to put on basically whatever you want.