You should do some research as explaining these over here could be quite difficult. But to answer your question, I guess the main considerations are required ADC resolution and maybe conversion time(more critical when it is an internal ADC module of a microcontroller, less important), and also the interface (serial I2C/TWI, SPI, parallel, etc).
Dawnlo,
ADC selection is a complex subject. The following articles should get you started:
. **broken link removed**
. **broken link removed**
Google "Bonnie Baker" ADC. She has many short articles on various aspects of ADC selection.
Regards,
Kral