There are a few different types of mods that you can use to vape. Mods can come with either an internal battery (which comes pre-installed in the Mod and can't be removed) and Mods that take external batteries (the most common external batteries that are used are 18650).

If you have to use external batteries to power your vaping needs, its important to be safe while using them. A new battery will come pre-wrapped in a battery wrap (battery wraps are the plastic outer casing that "wraps" the battery), this covering protects the battery from short circuiting and providing power to places it shouldn't be. If this protective casing is damaged in any capacity it's important to get the wrap replaced before using it again.

If your batteries are dropped please check for any dents or damage on the battery, if you find any, please dispose of that battery immediately in the correct manner.

If you are building on a rda/rtda deck and are using your homemade coils please make sure you find out the resistance of the coil/coils your using and what the continuous amperage drain will be on your batteries. The last thing you want is for your battery to start venting because you might lose your mod.
It's aways good to know of these things that can happen while using batteries, but don't be put off using mods that need external, with proper care you don't have to worry about your batteries.