What to write into log file?

My question is simple: what to write into a log. Are there any conventions? What do I have to put in?

Since my app has to be released, I'd like to have friendly logs, which could be read by most people without asking what it is.

I already have some ideas, like a timestamp, a unique identifier for each function/method, etc.. I'd like to have several log levels, like tracing/debugging, informations, errors/warnings.

Do you use some pre-formatted log resources?

Thank you