Launch DLC's are downloadable contents(DLC) such as costumes, additional character, map packs, mini games or prequels available for a limited time which makes you purchase the game on day one. Wherein such DLC's can only be seen in collectors edition.
Example:
Mass Effect2 - Free Terminus Gear at launch. After February 23, you will need to pay in order to use it.
On the other hand online pass is a game specific one-time use codes to be used for playing multi-player online. Purchase a used game with redeemed online pass (yey! It's cheap!) but the bad news is! you'll have to pay an additional $10 if you want to play online.
Example:
Need for Speed: Hot Pursuit - The online pass grants you access to online multi-player modes plus, two exclusive cars (LP-570-4 Superleggera and LP-550-2 Valentino Balboni Lamborghini's) and more storage space for your in game photo's.
This move is to discourage gamers from purchasing a used game or waiting for a greatest hits version of the game which is way cheaper.