extended vs charge battery -Is there a dif??

Android Land:
Need some help with the following:

- extended battery vs recharger are they the same? It looks like they are or am I wrong.

- what do all these numbers represent? amh or mah? any website out there that explains this?

- what should I look at in my phone so that I am choosing the right one. I have a Samsung galaxy express (AT&t) and have a Samsung galaxy tab 2.

I would like to extend the battery if I run out when traveling. Also do I need to look at some rating? As you can see I have read some posts but still confused and appreciate any info for educational purposes.
I also want to be able to use the phone when I attach the extended or rechargeable battery.

Important - want to make sure I get something that does NOT overcharge as well.

Thanks!
ladyandroid