Single crochet two together, a decrease. Pull up a loop in the next stitch, then pull up a loop in the one after, so you have three loops on the hook. Yarn over and pull through all three. Two stitches have become one and your count drops by one.
The mechanics are the same: several partly worked stitches closed together at the top. The difference is where they go. A dc2tog is worked across two different stitches and decreases your count. A cluster is usually worked into one stitch or one chain space, so the count does not drop and you get a solid textured bump instead.
Chaining four and joining leaves a fixed hole in the middle that you cannot close later. A magic ring is a loop you work into and then pull tight by the tail, so the center shuts completely. For amigurumi that matters, because stuffing shows through the hole.
Both are worked around the post of the stitch below rather than into its top. A front post stitch pulls that column toward you and a back post stitch pushes it away. Alternating them along a row makes the columns sit at different depths, and that ridged surface is ribbing.
Designers invent shorthand for stitches they use a lot, especially in amigurumi. A pattern that does this should define it in a key near the top, before the first round. If it does not, that is worth asking the designer about rather than guessing, because a made-up abbreviation cannot be looked up anywhere.
The left-hand column is US, which is what most published patterns use, and the UK equivalent sits underneath wherever the two differ. If you have a whole pattern to convert rather than one term to look up, the UK to US converter will do the entire thing at once.
A pattern written in shorthand you half know is a pattern you read twice. Wovely keeps your patterns in one place, marks off rows as you go, and opens on your phone at the row you actually stopped on rather than the one you think you stopped on.