The Spread is something like "Cardinals by ten." If you take the Cardinals and the spread, and the cardinals win, but by less than ten, you loose. So if you think the Cards will win but not by as much as the spread (I think this is the same as the line - "Cards by ten" - but I could be wrong), you can take the other team plus the points.
The other thing is the "over-under" which is expressed in a number - say 36. If you think the total points scored by both teams combined in the game is over 36, you bet over. If you think less than that many points, you can bet under.
Basically, the negative number is the favorite and the number tells you how much you have to put at risk to win $100 if that team wins (so, in this case, you have to bet $240 on Steelers to win $100). The positive number goes to the underdog and the number tells you how much profit you'll get for every $100 you bet (so, if you bet $100 on the Cardinals and they win, you get $200 profit).