Shirk surely is whatever stops you from fulfilling your obligations to God or anything which you rely on where you should rely on God..
.
Shirk in itself can be divided into three types:
- Ash-Shirk ul-Akbar (The major shirk)
- Ash-Shirk ul-Asghar (The small shirk)
- Ash-Shirk ul-Khafie (The hidden shirk)
The major shirk is the main one, which takes us out of 'the fold of Islam':
"Verily, Allaah forgives not that partners should be set up with him in worship, but He forgives except that (anything else) to whom He pleases, and whoever sets up partners with Allaah in worship, he has indeed invented a tremendous sin." (Nisaa, 4:48)