Common Units for Spaces in LaTeX

There are many places where we need specify exact spaces in LaTeX, for instance, inserting an image, arranging the position of several tables, etc. Normally in the template codes on Internet, people will find units such as "cm" and "in", which stand for "centimeter" and "inch" respectively. Then are there other units? And if yes, what do they mean?

1. in - inches 
2. mm - millimeters 
3. cm - centimeters 
4. pt - points (about 1/72 inch) 
5. em - approximately the width of an "M" in the current font 
6. ex - approximately the height of an "x" in the current font

Reference:

http://www.math.wvu.edu/~mays/696/latex_picture_env.pdf