Netherrack is a block that is found in abundant quantities in the Nether, its quantity is similar to that of stone or dirt in the Overworld. It is a frail block that breaks quite easily and has little blast resistance.
A block of netherrack (via screenshot)
Since netherrack makes up most of the terrain in the nether it can be procured by just going to the nether and mining some. It is also generated as part of ruined portals in the Overworld and Nether. Netherrack can only be obtained by mining nylium with a tool that isn't enchanted with silk touch.
Blocks of netherrack and nylium in the nether (via screenshot)
Nehterrack is used as a building block to help the player traverse the Nether because of its abundance and fragile nature making it easy to mine in large quantities. Netherrack can be smelted into a nether brick which is used to make nether bricks.
Smelting netherrack into a nether brick (via screenshot)
Netherrack is basically the dirt block for the Nether and it can be found everywhere and is easy to mine because of its fragility. Players use this to make temporary structures such as towers, bridges, and steps to reach places in the nether easily.
A. If a fire is lit on the top of a netherrack block it will burn indefinitely.
A. Netherrack can be used to traverse the nether and craft nether bricks which can be used for making fences and stairs.
A. No, smelting netherrack will not give the player XP points.