Take this card for example (follow the jump)....
http://lotrtcgwiki.com/wiki/lotr01002Always look to the bottom right hand corner for card information. R is a Rare, U is an Uncommon, C is a Common. Obviously, those go in descending order of "value".
The first number you see (in this case, 1) refers to the set number. There are 19 sets, as shown at the top of the page. If you click on the number at the top that corresponds to the first number on the card, it will tell you what set it is from. 1 being Fellowship of the Ring, 2 being Mines of Moria, 3 being Realms of the Elf Lords, so on and so forth.
The second number you see (in this case, 2) refers to the card number in that set. Each card is numbered according to culture, and then alphabetically within that culture.
Generally, rares are more valuable, and are "better" cards. There are some very useful commons and uncommons, but that's only evident in active gameplay. If you have any questions, you can always type the card title into a post here, and it will automatically link it to the card in the TCG Wiki, so that we as the forum community can better help explain things to you.