Cardiovascular Surgery (or more commonly, Cardiothoracic Surgery [assuming you want to operate on hearts and the chest; if you want to do peripheral and some central vascular disease, you want the specialty known as Vascular Surgery]) is a fellowship completed after 5+ years of a General Surgery residency. It is typically 2 years in length after GS.
Competitiveness still remains high despite the inroads Interventional Radiologists and Cardiologists have made into the field. At more high profile programs, research is still expected to be a competitive applicant. That's about the extent of my knowledge, so I'll allow others with more info to post their thoughts on the subject.
BTW, Vascular Surgery is also a post-GS Surgery fellowship.