Do dogs wag their tails only when they are pleased to see somebody:confused:

Tail wagging is more a sign of excitement. Many dogs are very friendly and excited and happy when a stranger comes. Wagging their tails shows the excitement. Unfortunately a few fear strangers. They may be wagging their tail because they are excited, but they are nervous and may decide they need to attack the dangerous intruder first. Body position is much more important. Stiff legged is bad. Ears back is bad. A wrinkled nose is bad.

I have noticed when I catch my puppies into something they shouldn't be, they wag just the end of their tails very rapidly. That is not a sign of being happy. It maybe trying to please.

Hi,
The other answer you received before this one is very good.
No, it's not always to please someone.
If you would like to read a little more, here is a link:
http://www.chacocanyon.com/pointlookout/031105.shtml

The link compares dogs' feelings to similar human feelings, and is some good reading!
I agree with the dog tail wagging as meaning excitement. Trick is, come to know and how to read the other signs given by the other answer, and some of those are in the link also.
I do wish you the best.

Dogs wag their tail for many different reasons, as Labman stated it is usually happiness or excitement. But if you do not know the dog it may not be a happy wag, as all dog were once wild and have been domesticated by humans, they still retain some of the wild characteristics of the pack animal they once where. Wolves and dingoes and other pack animals will communicate their intention with body language such as waging tails licking, noises and posturing to express their dominances or submissiveness, in a hierarchy of a pack. Fredg has given you are very good page to read, I would also like to leave you with Wagging Tails (http://www.peteducation.com/article.cfm?cls=0&cat=1276&articleid=1158) by a site called PetEducation.com (http://www.peteducation.com/default.cfm). I hope that they will in some way be helpful to you.

Technically dogs wag their tails because with any emotion they release different hormones through their anus it goes back to their wild instincts. The hormone that is released is spread from the wagging of the tail. Just like when a dog tends to lick you more in the face than anywhere else is not because he's giving "kisses" it's a wild instinct when they lick their mothers face for food. :)