Well, yes, of course, since they don't mean the same thing.
De (pronounced: dom) = they
Dem (pronounced: dom) = Them
Dessa = these
"De här" (pronounced: dom här") is more common than "dessa" though.
"De" can also mean "the", but we only use it if there's an adjective before the noun. "de" is for when the noun is plural and "det" or "den" is for singular.
The red balls = De röda bollarna
The red ball = Den röda bollen
The balls = Bollarna
Balls = Bollar
Ball = Boll
Oh, and with "dessa" you don't have to make the noun definite.
These balls = Dessa bollar
These balls = De här bollarna