This pages describe the basic concepts and syntactic constructs of ASN.1 may be in some funny way, just to get on with all that notation without grudgery. It contain lots of cross references, so if you can't remember something, just click that underlined text. Following pages are:
Getting through all that you can return to main page of ASN.1 and read more accurate definitions unless you're the happy one, who has not to use it...