This phrase is often seen on invitations to house parties, it means Bring Your Own Booze (booze is a slang term for alcoholic drinks). This means that the host of the party might be providing food or snacks, but that guests should bring their own drinks. It is often spoken too:
“I’m having a party at my place, it’s byob but I’ll be ordering a few pizzas so don’t worry about food.
This is a common expression that means “For Your Information”, and is used to point out some information the speaker thinks would be helpful to the listener.
We usually use it for something that the speaker thinks is important, but isn’t sure if the listener is interested or not:
“My coworker is really pretty, and FYI she’s single.” (This implies the listener might want to ask the coworker out on a date)
Or we use it angrily sometimes to challenge someone with new information:
“Yeah I don’t have a smartphone, but FYI: your phone was made by poor sweatshop workers and you should be ashamed.”
This is another very common one that means “By the way…” and is used to give some additional information to someone that either isn’t very important:
“By the way, I wanted to ask if I could borrow your powerdrill sometime?”
or something a bit off topic:
“Yeah I saw that movie too. By the way, did you know that director used to be a doctor?”
It is never spoken as BTW, and is always said in full.
This expression means Best Friends Forever (e.g. “Oh yeah I love Sandy, we are BFFs”), or Best Friend Forever (e.g. “Have you met Sandy? She is my BFF.”) The term “Best Friend” is used somewhat lightly in English, so this phrase was invented to show a stronger relationship. But now it is being used very commonly too, so maybe we will need a stronger phrase soon?
This means “Boyfriend”, and is usually only written, never spoken. It’s a pretty simple one, but I’ve seen some English learners write things like “I had dinner with my BF” and think that it means “Best Friend.” Just be aware that BFF and BF have completely different meanings