Minecraft (Pocket Edition)
Lava is found virtually everywhere in Minecraft, both above and below ground. It is found in lake form, puddle form, and in lava waterfall form.
The best way to obtain Lava is with the use of a Bucket. If you get near the Lava (not too close, now), you will be able to collect the Lava by tapping on it. This will fill your empty bucket and give you a Lava Bucket.
Usage / Purpose
Lava also makes for a pretty nice looking moat.
Lava can be dumped from a Lava Bucket almost anywhere. It can be used as a difficult to manage weapon (most creatures lack fire resistance).
Lava can also be turned into Obsidian by dumping water on it.
Lava cannot be consumed as a nutritious, albeit scorching hot, snack, despite what your friends tell you. Lava also cannot be used to clean your cherished Diamond Sword by throwing it in a Lava lake.
Lava will catch other blocks on fire, such as Grass Blocks and Trees, creating forest fires. This, unlike the above statement, is true. The image below is from natural Lava flow catching stuff on fire.