TTYL is simply an acronym which means Talk To You Later. The acronym Talk To You Later is mostly used by people when chatting online with their friends. The expression can be used to tell someone that you no longer have their time either because you are going offline or you want to attend to some things that really matter, so in a bid to inform the person they are chatting with, they will type TTYL.
You will see that, knowing at least a good number of online acronyms at times can be a bit helpful. There are so sentences you do not need to write in full because there are acronyms for each of them. When you type the acronym, you safe more time and it makes chatting more easier. However, one needs to be careful at the use of all these acronyms so we won't use them when writing or engaging in any formal communication.
There are a lot of people who know that TTYL means Talk to You Later. This can be used by people who would like to end the conversation for the meantime. You may use this when you are chatting with someone and you remember that you still have to do a few things.
This abbreviation can also be very vague because there is no specific time when you are going to talk with the person again. There are other abbreviations that may be used more because they are more specific. For example, TTYT means Talk to You Tomorrow. This will let the other person know that he/she can rely on you to chat again the next day.
TTYL is an acronym. It is commonly used in texting, especially amongst teenagers, and was used as the title for a novel aimed at this group. It stands for Talk To You Later, and can be used as capitals or lower case. The latter is more polite. Ttyl is part of a whole slang vocabulary endemic in texting language.