Lava is an uncertain presence in Minecraft- while it’s always going to be there, whether that’s a good or a bad thing is a constant question. Molten rock can be a hideous hazard if you’re uncautious or even just a touch unlucky- but it pays immensely to know your way around this fiery liquid, and with that in mind, it’s time to look at lava.
Lava behaves, in some ways, a lot like water. Lava comes in source blocks and flows, and only source blocks can be moved- flows will simply ‘run out’ without the source blocks supplying them with more lava. Lava, however, flows only five blocks to the seven that water reaches before running out. Curiously, though, while water flows only straight down, lava will always flow out to the side from a source block, no matter whether or not there is anything to flow on top of. Also, lava flows very slowly, taking a good second or two to extend the flow for each space that it will reach. Of course, lava won’t push things along, but that’s mostly because it tends to immolate said objects instead of supporting them.
Lava is basically found in two places- underground, and the Nether.
Lava in the Nether is everywhere. The Nether has no water in it unless a player brings some there- it is a dry and dare I say it, hellish environment. The Nether does, however, have immense lakes- oceans, nearly- of lava pooling in it, and pouring from the walls and occasionally ceiling. While this does the favor of ensuring that the Nether is well-lit, it also provides a constant hazard- there will be no safe suns or runs on the beach in the Nether, and construction over ‘water’ is at best hazardous and at worst foolish.
Lava in the World is much more reclusive. Typically, you only find lava in the World deep underground, in the midst of the stone layers. It can be exposed into flows and pools by caverns, mines, ravines, and dungeons, but mostly it is in sealed masses deep underground. There are exceptions, though- because of the way the map treats areas under mountains, many cliffs will have ‘underground’ water or lava creating long falls out the sides of them. Leaping into a ravine may wind you up saved from falling-damage death only to burn instantly to ash in a pool of lava down below. And, interestingly, sometimes small lava pools will be found on the surface of the World, bubbling merrily away on one coastline or another (Supposedly this may happen inland too, but I’ve only ever seen it on the coastline).\
Lava is, however, still a hazard in the World. It is one of the reasons you never dig straight down nor straight up- falling into a lava pit by surprise is not only fatal, but destroys your items when they also fall into the lava as your dying body drops them. Similarly, suddenly creating a lavafall onto your own head is fatal and destructive. Despite this, lava is very important to mining in the World. What’s worse, with recent updates (1.2), lava occasionally spits out tiny short-range fireballs that can set you on fire. Ouch.
You see, Lava draws ore. Where you find lava, there will always be much more plentiful veins of metals, diamond, redstone, coal, iron, and lapis. Diamond and redstone are especially good reasons to seek out lava pits- and to always dig out the area around them. Neither of these is a resource you want to ever skip an opportunity to find if you can possibly manage it.