Yep - cocktail attire is dark suit for man and cocktail dress for woman. How formal the cocktail dress is depends on venue/occasion. It can be as simple as an LBD with heels or a formal cocktail dress.
Think "Mad Men". As lcw589 says, men should wear a simple dark suit, a shirt & a tie with dress shoes. For women, the dress is short (i. e. not floor length) and usually as a little sparkle or shine to it. A dress with a mathcing or coordinating jacket is great if you might be faced with cooler evening temps or heavy AC. If you have any glittery jewelry, you can break it out. Vintage/used clothing stores are a great place to pick up something fun without breaking the bank. If you are going to go out and buy something, and want to be able to use it again, the proverbial "little black dress" with some rhinestone jewelry is a classic choice.
Most of all, make sure your dress and shoes are comfortable! You want to enjoy the party!
Little black dress (at or above knee), strappy heels, pearl or diamond necklace and earrings.
I only deviate if I know the location and the people, i.e. work functions that say cocktail attire I will often wear a pencil skirt, closed heels, and a button down or sweater. A family wedding that says cocktail attire warrants more color, and maybe a little sparkle or flounce.