Squadron 42 can be played in single player. It's kinda like the game Wing Commander. Star Citizen is the persistent online universe with real people flying around exploring, fighting, mining for materials, shipping materials and resources. Basically do whatever you want. It is scheduled to hopefully release in 2015 with many alphas and betas before then.
So in short yes it will be an online game.
There's also private moddable servers to fuck around with, if you don't wanna be part of the one big persistent universe.