General Cards fellowship is the same as Cardiology or Cardiovascular Diseases. It is 3-4 years in duration and teaches one to be proficient in ECHO, all forms of stress testing (+/- nuclear techniques depending on where you go), cardiac intensive care, consultative cardiology, electrocardiography (of course), pharmacologic EP, diagnostic catherization (angiography, left and right heart cath, intensive care monitoring, other catheter based procedures EXCEPT INTERVENTIONAL ones). There is other stuff, but I've mentioned the big ones.
A general Cardiologist is either INVASIVE (the catheter based stuff I described above) or NON-INVASIVE (focuses on ECHO and non invasive techniques of risk stratification, but does not do diagnostic cathetertization).
You can do fellowships in:
CHF/Transplant -- 1-2 years, focuses on management of CHF pt and usually encompasses Transplant Cardiology training. As you can imagine, most of these guys are academic or are affiliated with one of those big heart hospitals with transplant programs
Interventional -- 2 years, with SMALL minority of 1 year spots. This teaches all cardiac (PTCA, rotablator, TMLR, valvulopasty, carotid interventions, and so on and so on) and most peripheral interventional techniques.
ECHO -- 1 year, just like it sounds
Nuclear -- often combined with ECHO under a "non-invasive" fellowship or separate, teaches more in depth the various diagnostic nuclear modalities like Ad Thal, MUGA, etc.
Preventative -- 1 year, just like it sounds, usually with lots of Epi and biostats teaching.
EP -- 2 years. Most places allow you to fold in your first year of EP into your final year of general Cards. The orthopods of Cardiology (in terms of "they kind of forget everything else they learned")-- pharmacologic and procedural EP (mapping, ablations, surigcal implantation of ICDs and pacemakers). Very esoteric but very cool stuff. In demand now because of studies like MADIT-II and others.
There are more specific details to each, but you get the general idea.