When you're bragging, you're boasting. If you can't stop talking about your fancy new shoes, that's bragging.
Ever notice how some people are full of themselves and constantly telling you about the great things they've done? Those folks like to brag. Bragging is a verbal kind of showing off. We all do it sometimes, and it's almost always annoying when other people do it. A specific boast can be called a brag. Athletes often brag before big games. But only the winners can brag after, because they backed up their words.
