There's 4 airports:
LAX, approx. 35 miles away: pros, often the cheapest and has the most flights and times available, Disney Resort Express bus runs on a frequent and convenient schedule. Cons, can be expensive for ground transportation because its the farthest away; its your typical huge, dirty, loud international airport.
ONT, Ontario: approx. 30 miles away: pros, often fairly cheap to fly into, not as big. Cons, ground transportation can be pretty expensive since its quite a distance.
SNA (also known as Orange County or John Wayne airport) approx 12 miles away: pros, the closest airport, small, yet very nice and convenient.
Disneyland Express also runs here. Cons: its usually the MOST expensive of all the airports. Dont know why, but it costs us over $100 or more extra, per person, to fly into SNA.
LGB, Long Beach, approx 14 miles way: the choice of many. It is a small, clean, quaint airport (you deplane down stairs onto the tarmac!) and nearly as close as SNA; yet it can cost considerably less to fly into. Cons: ground transportation can cost more here; Super shuttle is 3x as much for some reason, although a taxi will cost you the same as from SNA ($40 flat fee one way). Disneyland Express doesnt operate from here.
Hope that helps!