Booster Gold is a guy from the future who comes to the past (now) to be a hero and become wealthy. Yes, he really is a superhero with the Justice League, it's just that he's mainly used as comedic relief.
Blue Beetle, is a teenager that gets alien technology bonded to him that can do all kind of cool things. Jaime Reyes is the second Blue Beetle, the first being Ted Kord (who will also make an appearance in the episode) who was kind of like Batman, only with a Beetle fixation instead of Bats.
So who's playing who? Cast as Booster is "Days of Our Lives" actor Eric Martsolf.
Blue Beetle is being played by Jaren Brandt Bartlett.
And finally, Ted Kord is being played by Sebastian Spence.
There you have it, kids. It looks like Smallville is trying to introduce as many DC characters as possible (excluding GL, WW, and Bats) before they close up shop. Maybe they'll even have a cameo from Plastic Man (I can only hope) before they go.