The casual writer is free to let his or her imagination flow and write as he or she would speak, without having to stick to a particular format. Sentences are mostly short and direct, with no further explanation needed.
College writing, mostly in the form of essays, on the other hand, is usually required to adhere to a particular style, such as MLS or APA, for instance. This mostly depends on the relevant class subject.
As a rule, it is done for a specific purpose, for example to prove a thesis statement, and the generally pre-determined content will require research. Sources of included information must be cited.
The information must be written in a clear, to the point way, and certain formal requirements and a pre-determined structure have to be conformed to. Often passages or sentences have to be elaborated on to make specific points.
The finished paper has to be proof-read in order to eliminate grammatical or typing errors, as these form part of the grading process.
Casual writing is therefore different to college writing in content, with casual writers being free to choose their subject, whereas college writing has pre-defined subjects; as well as formality, structure and style, with college writing being tied to specific requirements, whereas casual writing can be done in any style without a particular format or structure having to be applied.
Another difference lies in the fact that typing errors or grammatical mistakes do not matter greatly in casual writing, whereas these kinds of errors or mistakes within college writing may lead to down-grading of an otherwise perfect essay.