Spiders are common neutral mobs that have the unique ability to climb walls, which spawn in the Overworld in 3×3×2 space on solid blocks (centered on the middle block) at a light level of 7 or less, in groups of 4. The top blocks can be transparent, but not solid. In Pocket Edition, leaves can spawn spiders, provided that there are the same conditions as above.

What it Drops

Spiders drop 0-2 string upon death. If killed by a player or tamed wolf, it also has a 1⁄3 chance to drop a spider eye. They drop 5 experience when killed by a player or tamed wolf.

Spider Jockey

There is a 1% chance that a spider will spawn with a skeleton on its back, forming a spider jockey.On hard difficulty, spiders have up to a 10% chance (depending on regional difficulty) of spawning with a single beneficial status effect. This effect can be speed (40%), strength (20%), regeneration (20%) or invisibility (20%), and has an essentially infinite duration.

Here's where I got the information:)