just some of the characters are different,they have the same pronunciation ,but different way of writing

Traditional Characters: http://en.wikipedia.org/wiki/Traditional_Chinese_character
Simplified Characters: http://en.wikipedia.org/wiki/%E7%AE%80%E4%BD%93%E5%AD%97

Personally, I prefer Traditional Characters, but Simplified Characters seem easier for Western learners.

They are only different in shape

they are the same in pronouncation and different in writting.
the traditional one have more strokes and they are mostly used in Taiwan province in China now. Also, they could be found in some old books.

There were simplified Chinese characters hundreds years ago. For example, calligrapher created 台 in Ming Dynasty but some people still use 臺 in Hong Kong, Taiwan and they believe it's the correct writing.

Some Chinese thought some Chinese caracters are too complicated to write, and simplified them.
People in China now use simplified Chinese as daily language, except in Hong kong and Taiwan.
However, if you want to read the old books written by ancient Chinese writers, you'd better read the traditional Chinese caracters, otherwise, you may be misunderstand.