Hrqls: Additional point:
So playing in your own territory costs you a point. Sometimes you can make your opponent playing in his own territory by threathening to cut some of his stones of of killing his group.

Hrqls:
is it always true then when there is an even empty spaces within a group, then the group is not dead ? if it would have been an odd number of spaces i think the group would be dead ?

No and no. A group needs two eyes to live: having two eyes prevents the opponent to capture the group since he's not allowed to fill in both eyes at once.
Whether the group can make two eyes or not depends on the internal shape of the group and on whose turn it is. Some shapes with 4 empty intersections are alive, some with 6 empty intersections are dead (when it is the opponent's turn).
For exact shapes see the tutorial, since it is difficult to explain by words only. The tutorial is a couple of interactive lessons.
www.goproblems.com is also a good excercise even for trained players.

Hrqls: To surround territory is better: you can get more points with less stones.

In the beginning: claim a empty corner on the 3th or 4th line. Moves on the 1st and 2nd line are nearly always bad untill the endgame.

When attacking a stone you are one move behind, so that's a dangerous strategy.

And: opponents are allowed to have territory also, but not more than you have

onigoroshi: Yes, it does. If you are a strong player, and losing from a weakie, you'll lose more points than when losing from a master. And vice versa of course.