[iyulaev]

What is the difference between kill and kill -15?

Well, for one, the second isn't syntactically correct.

I can't decide what's worse, brainteasers or brainless trivia questions.

[spudlyo]

Pedantic nonsense. Most of the time when you're using kill you're going be using the bash shell built-in, where it is syntactically correct.

kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] ...

Even so, with /bin/kill -sigspec is still valid and common usage, even if it is not documented in the manpage.