Players in FIFA Ultimate team are split into three categories. Bronze players rated 0-64, Silver players rated 65-74 and Gold players rated 75 to 99. These generally reflect their ability in real life and the price reflects their popularity within the game. You can create a team mixing any of them but some competitions offline and online might have to meet certain requirements.
Each category is further broken down into Normal, Rare and In Form. The following image shows one of each for bronze, silver and gold. Normal is a standard card, rare means it is just that and more valuable.
Every week in real life some players are outstanding. Ultimate team on FIFA recognises this and creates new versions of the players with increased better stats. These are called In Form players and become available to buy and are inside packs during each week. Sometimes in different positions to their original card. If you have a normal version of a card that does not change to the in form version. It means there will be the normal version of the card available and also the In Form version.