I've played chess since I was kid and after much practice have become a decent player. I still though, have little strategy to my opening moves besides starting with a King's Pawn Game. I know professional players and grandmasters have a large repertoire of openings they know. They also know a large number of responses to those openings. How important is knowing different openings when playing chess casually?
Its far better to thoroughly understand a small number of openings than it is to mindlessly memorize a large number of openings. Frequently, if you know why a set of moves is considered optimal, then you're in a better position to adapt once the board gets "off the opening". However, if you have a wide set of openings memorized, you may find that while you are moving down your 'optimal opening', ready to transition to a strong midgame, your opponent may have other plans, and make a move that takes him off your chosen opening.
Now frequently, there's a good reason why that move is not covered by your opening chart; that move may be terrible. But if you don't understand why that move is terrible, then you may not be able to take advantage of it, and if the other player improvises better/faster than you, you may find yourself at a disadvantage despite your 'superior' play to this point in time.
Don't let that happen to you.
1000x more important (and more fun!) than even studying theory is.. improving your tactics! The best place I know for this is chesstempo.com.
You can easily get up to 1700~1900 USCF never learning any theory at all (casual players are usually 800~1400), and I've even met a few players over 2000 (master level) who have never studied any theory!
However, if you insist on studying theory, learn the barebone basics, then study endgames - knowing an opening might give you a slight theoretical edge going into the middlegame, but knowing an endgame could mean the difference between winning and drawing/losing. Remember, the winner is the person who makes the second-to-last mistake!
For casual play, I don't think memorizing chess openings is all that important. However, studying and understanding why some counter-moves are appropriate for this or that opening may prove quite useful.
Chess openings are very important if you are interested in winning games, as they are sets of moves that help develop your pieces quickly and gain some control of the centre of the board. Having a grasp of an opening sequence that your opponent doesn't know should give you an edge in the early game that you may be able to turn into a clear lead during the middle game.
On the other hand, if you are playing against an opponent who doesn't know the opening then you may quickly find yourself in an unexpected position - so try to concentrate on learning a shallow amount of an opening.
Finally the other thing to say is that some openings are a must know. See for example scholar's mate!
Opening memorization is not important at all, especially to casual play, and shouldn't be the focus of study time for serious players, either, even up to around expert/master level. For casual play, you should instead follow basic opening principles: put a pawn or two in the center, move all your pieces once before moving any piece twice (unless there's a tactic), castle. For serious players, it's worth playing a few openings and learning them well, slowly building your mental tree of opening variations game by game, by playing a game and looking up the official lines afterwards, to see where you diverged.
I once spoke with a well known local expert player, who confided to me that he wished he could "get all those years back" that he spent studying openings. I asked "what would you study instead?" He instantly responded: "tactics and endgames." He usually played unorthodox gambit openings and almost always built a great attack with the initiative.
You don't need to memorise every opening move and variation to be able to play it - not for casual play at least. It's better to just play and get a feel of the different characters that varied first moves and common responses can give a game. You may find you enjoy losing games trying to crack a c4 first move more than winning with a King's Pawn game, and you'll learn to understand the opening better than you ever will by memorising the first X moves.
And let's face it, for most of us the game isn't won or lost in the first few moves; there are plenty more opportunities for middle and end-game blunders. A world of experimentation and catastrophic strategic mistakes awaits!
In casual play, it's not necessary to memorize openings. Because your "casual" (by definition) opponent won't know them.
It's much more important to know tactics, and to "read" the opponent's moves. If s/he departs from the "book," its usually to that player's detriment. Occasionally he'll stumble on an alternate variation, rarely find a new line. Figure out why your opponent's moves are (probably) wrong, and take advantage of that fact.