They have a few friends tat help them such as Isabella (who always says "Watchya doin?) and the fireside girls, Buford the bully, and Baljeet the Indian kid who obsesses with school.
Phineas has a head shaped like a triangle and a single tuft of orange hair. He always says "I know what we're gonna do toaday!" and "Wheres Perry?" Ferb is British, doesn't talk much (about one line every episode), has a square head, and has green hair.
Their older sister, Candace, is obsessed with two things and two things only: Busting her brothers, and Jeremy, her crush and eventually (spoiler alert!) her boyfriend. He is amazingly cool to be able to put up with her.
My favorite part is the intercepting yet completely different plot of Perry the Platypus, or Agent P, Phineas' pet. Agent P is the nemesis of Doctor Heinz Doofenshmirtz. Doctor D is always creating these "evil" machines which are really rather silly. The names of said machines always ends in "-inator," such as the shrink-inator or the really-really-bad-inator. You get the picture.
Perry always storms into Doctor D's apartment (or other things like blimps or space stations), usually breaking through a wall (or a window or the ceiling or a door...). Doof always says something like "Perry the Platypus. So glad you could make it." He then says some sort of bad joke or pun, and then traps Perry in something absurd. He explains his whole evil plan, then Perry escapes and ruins it. Doof usually gets hurt, then yells out "Curse you Perry the Platypus!!!"
And in conclusion, my favorite cartoon, Phineas and Ferb, is totally awesome. The End.