Colloquialism, also called colloquial language, is the linguistic style used for informal communication. It is the most common style of speech, most commonly employed in conversation and similar informal contexts. Colloquialism is often characterized by the usage of interjections and other such devices. It uses non-specialist terminology, and the words used keep changing on a regular basis. Colloquialism can also be distinguished from formal speech by its usage of expressions that can often have illogical syntax.
However, this does not mean that colloquial language is non-standard or slang; it simply means that a different expression is preferred in more formal contexts. That being said, some forms of colloquialism do contain some amount of slang, sometimes even a considerable amount, but it still does not mean that colloquial language is synonymous with slang.
Colloquialism should also be distinguished from non-standard forms of expression. The differences between standard and non-standard expressions are not exactly connected to the difference between formal and colloquial language. Colloquial, formal and vulgar language are more of stylistic variations and diction differences, and hence can be considered to exist outside the standard and non-standard dichotomy. However, the term colloquial is also equated with non-standard at certain times, but only in certain terminological conventions and contexts.
From a philosophical standpoint, colloquial language is considered as ordinary natural language, which is distinct from other forms of formal language used in logic and other prominent areas of philosophy.
Colloquialism is distinct from slang in the sense that slang is used only within certain communities and is quite localized in usage. It is also distinct from jargon, in that the latter is used specifically within a certain profession, trade, industry, activity, etc.
So it can be said that colloquialism includes slang, abbreviations, idioms, and other informal words used by most native speakers.