Power only applies to creatures. This is because power is used for combat (and in some cases, for spells like Soul's Fire), in most cases. Power determines the amount of damage it can deal to another creature, planeswalker, or player. Let's put it in easy terms.
Each creature has power and toughness. Whenever a creature attacks and it gets blocked, it deals whatever number of power it has to the blocking creature's toughness. Example: A 4 power and 4 defense creature (4/4) attacks, and a 2 power and 5 defense (2/5) blocks it. When you take a look at a Magic Card, at the bottom right, there are two numbers. The number on the left is the power, and the number on the right is toughness. So if a 4/4 attacks and a 2/5 blocks, the 4 power goes to the 5 defense of the blocking creature. Since 5 minus 4 does not equal 0 (it equals 1), the 2/5 does not die. And since the 2 power of the 2/5 goes to the 4 toughness of the 4/4, and 4 minus 2 equals 2 (and not 0), neither of the creature die. (You can, however, play another spell to finish off a creature that has a lower amount of defense after combat.)
Here is another example. A 3/1 creature attacks, and a 2/2 creature blocks. The 3 power of the attacking creature goes to the 2 defense of the blocking creature, and since 2 minus 3 equals -1 (you always subract the power of the creature from the defense) and therefore is less than (or equal to) 0, the 2/2 dies. But since the 2/2 also gets to deal its power to the 3/1, and 1 minus 2 equals -1, the 3/1 dies as well. (Some exceptions to this are creatures with abilties such as deathtouch and first strike.)