Mine's a little less simple, but only just.
Reactions can be done in response to someone else's action, even if it's not your turn. This includes things like the Reactive Counterspell feat or letting go of an item if someone tries to grab it from you. You can only perform reactions if something does something to you.
Free actions can be done during your turn while you're doing other things. This includes Quickened Spells, drawing a weapon with Quickdraw, or shouting up to 15 seconds of dialogue (I call it the "dramatic effect" paradox, which lets you say more in a 6 second combat round than would actually fit in 6 seconds; also, each person can lucidly say 15 seconds of dialogue each, which explains why in anime sometimes people spend a minute discussing their rival's strategies before counterattacking). You can perform a lot of free actions in a round, but the exact number is just left up to reasonability. Another free action you can do is move 5 ft. as an adjustment (not incurring an attack of opportunity), but only if that's your only movement in the round.
Move actions are movements, like normally drawing a weapon, moving 30 ft., or getting out an item from a bag.
Then there are standard actions, which are things like single attacks, casting a spell, or drinking a potion.
You can perform two move actions or a move and a standard action in one round.
Full-round actions are running, making full attacks, or casting long spells.
Oh, and I changed my summon spells now so they're like Magic: the Gathering. Summoning the creature is as fast as any other spell, but the creature has 'summoning sickness,' and can't attack until your next round.
