Can someone explain what a crystal's ppm stability rating means?

I need to buy some crystals (or oscillator modules, whatever's better) for an electronics project. If you look at the crystal on sparkfun it says it's rated at +/- 50ppm stability: http://www.sparkfun.com/commerce/product_info.php?products_id=535 The oscillator at electronics express says it's rated at... show more I need to buy some crystals (or oscillator modules, whatever's better) for an electronics project. If you look at the crystal on sparkfun it says it's rated at +/- 50ppm stability:

http://www.sparkfun.com/commerce/product_info.php?products_id=535

The oscillator at electronics express says it's rated at 100ppm:

http://www.elexp.com/cmp_osc1.htm

What does the rating mean? Which is better? If I get the oscillator module do I still need to decouple with capacitors?