Characters which have several different shapes
The Unicode standard put one character into one code point, but the character can be written differently in simplified Chinese, traditional Chinese, Japanese or Korean(oh, they don't use characters now, only Hangul). From the Unicode website, you can download a list of all characters, with their origin local standard and shape. A example for character "直" is shown below.



The number under the character shows the original local standard (and code point in the standard). The local standard from mainland China are listed here. In short, it begins with "G". And "G0" through "G9" (excluding "G4k") should be the characters more often used.

The shape from the mainland standard is used in simplified Chinese.

EDIT:

In some cases, purely typographical variants are assigned different code points. These are referred to as z-variants.