Basically from my own experience there are 2 main types: rust, or limescale/calcium
1. Rust-red/orange stains on sinks, toliets, clothing.
2. Limescale/calcium (which I currently battle) is a white-ish coating on fixtures that come in contact to water. More insidious is the way that it invisibly attaches itself to hair, contributing to build up, brittleness and drieness.
There are quite a few postings and articles on this site about that are extremely helpful.