College writing is the type of write that should be used in a professional aswell fromal or academic type of writing environment. This type of writing should contain proper language, spelling, correct grammar usage, and flawless sentence structure. Casual writing or "free style" writing. This type of writing is used when texting, emailing or sending personal and or familiar written communication style of writting. In casual writting you're less likely to find proper sentence structure and a higher chance of slang or relaxed type of writing. Also you dont have to spell the whole word out you can use things like brb for be right back or ttyl for talk to you later or lol laugh out loud things like that is always casual writting.