If you tend to brag about your accomplishments or flaunt your possessions, you're a show-off. If you make sure everyone in your art class notices your brand new laptop, they'll call you a show-off.
It's important to a show-off that everyone notices them. Whether bragging about their grade on the math test, parading their fancy new hat through the school hallways, or demonstrating their ability to juggle five oranges at once, a show-off really enjoys spectators. The verb show off came first, while the noun appeared around 1920, defined as "a person who makes a deliberate and ostentatious display."