A boyfriend or girlfriend is very much like a friend. They're someone you can talk to. Someone enjoy doing things with (cooking, playing video games, sailing, playing card games, watching movies with, jogging, playing board games, attending ComiCon, solving puzzles, whatever floats your boat). Ideally they're someone you get along with really well, who really gets you, and who tends to like a lot of the same things you like. They should respect you as a person.
The thing that differentiates them from a friend is the amount of touching between the two of you (more than with a friend), the type of touching between you two (holding hands, kissing, etc.) and their sexual appeal. The older you grow, the more you and your friends likely take into account and need to take into account whether your ideas about family, raising children, career goals, and whether you could live with them for any extended period of time. The reason is simple, the older you are, the more likely sex and marriage are to be considered as options.
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shorthand, a girlfriend/boyfriend is someone you're both physically and mentally attracted and emotionally attached to. Only mental attraction=friend. Only physical attraction=fuсkbuddy.
Good summary.