What sets kielbasa apart from other members of the sausage family is its coarse texture, heady garlic flavor and classic Polish preparation - it's traditionally not smoked or lightly smoked at best.
Smoked sausage, on the other hand, is cooked and then smoked. Often, artificial smoke "flavors" are added as well. Because they're fully cooked, you can eat smoked sausages cold or warmed. Other varieties include Italian mortadella, Cajun andouille, German bratwurst, bologna and hot dogs. Smoked sausage can be made from ground pork, turkey, beef or a combination of meats -- and they can include any variation of seasonings.
Polish kielbasa is traditionally made from ground pork. Some commercial variations of kielbasa are made from ground turkey, but these aren't authentic. Garlic, along with pimentos and ground cloves, are the predominant seasonings.
Another variation is dry kielbasa, which is smoked and then dried to extend its shelf life. Dry kielbasa is usually served cold on a cheese tray, but has the characteristic garlic flavor of regular kielbasa. The drying process makes the flavor slightly more intense and the texture chewier and denser.
So when the hankering for a flavorful, pork dish hits, go ahead and cook up a couple kielbasa. If a smokier, leaner meat is what you crave, a smoked sausage of any variety is the way to go.