Three different types of omega-3 fatty acids, and they aren't all created equal. Alpha-linolenic acid (ALA), occurs in plant foods like soybeans, canola oil and pumpkin seeds. The second and third types, eicosapentaenoic acid (EPA) and docasahexaenoic acid (DHA), found mostly in seafood, are most closely associated with boosting heart health. Since your body has to first convert ALA into EPA and DHA for it to be effective, and only a small amount actually converted, it's important to get the bulk of your omega-3's from fish sources.
References:April 1, 2011, Family Circle