a) If you've ever heard of 'Greek love' then you will easily understand bisexuality. The ancient Greek males would have sex with both sexes, some may have only had sex with women to procreate.
b) Liking both sexes in general, usually romantically and/or sexually. A mix between being gay and straight.
a) Alexander the Great was a bisexual.
b) Jim is dating Janie but he did date George before her.