Dialect:  language variety used by a particular group of speakers; mutually intelligible forms of a language that differ in systematic ways from each other.

Code-switching:  the insertion of a word or phrase of a language other than that being spoken into a single sentence, or the movement back and forth between two languages or dialects.

Borrowing:  the incorporating of a loan word from one language into another.

Broadening/narrowing:  the changing of the meaning of a word in time to be more or less encompassing than originally intended.  "dog" used to mean one specific type of dog but the term broadened; "hound" used to mean any type of dog but it has been narrowed.

Pidgin:  a simple but rule-governed language developed for communication among speakers of mutually unintelligible languages, often based on one of these languages.

Creole:  when a pidgin is taught to a new generation as their first language

Lingua franca:  the major language used in an area where speakers of more than one language live that permits communication and commerce among them.

Jargon:  special words peculiar to the members of a profession or group.  For example:  "Quiddich"  "The Snitch"  "The Quaffle"

Slang:  words or phrases used on a casual speech often invented and spread by close-knit social or age groups.

Register (style):  the type of dialect used in particular situations (regional and social dialects too)