Cardiovascular training, technically, is anything that trains your cardiovascular system. This typically means anything that elevates your heart rate. So, yes, most physical fitness training could be considered cardio.
But that's not likely what you're after. When most of us think of cardio, we think of endurance-type activities like running. And this makes sense because these type of activities lead to a SUSTAINED elevation in heart rate while continuing to perform said activity.
Cardio training generally focuses on developing our aerobic energy system. This is the energy system that allows us to function at an elevated heart rate for an extended period of time.
Compare that to anaerobic training (e.g. sprinting) which allows us to function at a peak heart rate but for a very short amount of time. This shouldn't be discounted as one of the best ways to develop aerobic endurance is to develop anaerobic capacity (don't worry, I'll simplify what this means and how to actually do it in a moment).
Endurance is a combination of both speed and duration. Most people want to focus on duration first and understandably so. Eventually, you may find yourself wanting to get faster for that longer duration. That's where anaerobic training comes in.
Like weight training, the best way to develop these different capacities is to progressively build them.
Running, rowing, or biking (stationary bike included) are some of the best ways to do this.
Using running as an example: start running at a comfortable pace for about a mile or whatever distance you can comfortably complete. On your next run, add a quarter to a half mile at that same pace. This might still be comfortable for you (meaning we haven't yet found our baseline) or it might be a little bit of a challenge.
Do the same thing on your next run until you find the last half to a quarter of a mile being fairly challenging. By fairly challenging I mean you could do it, but the thought of slowing down or stopping crossed your mind. From here, you'll likely either add shorter distances or sometimes no new distance at all on subsequent runs. It's going to take more focus and perseverance to push through the challenge of the added distance. The longer the distance, the slower you'll progress. But it won't take long to build up to some serious miles as long as you slowly add distance (otherwise you risk getting hurt, your body has to adapt too) and stick with it. Consistency will be key.
When you start wanting to focus more on speed, cycle in some shorter runs at a faster pace (i.e. half your normal longer distance at a 1:00+ minute mile pace faster) and some sprint work (a 4x50 or 3x100 should do it). Even if you don't think your sprinting is all that "fast," it will help you get faster for the longer distances. And it'll also help you get faster at sprinting!
The best way to get faster is to run faster!
The best way to run longer distances is to run longer distances!
The best way to do anything is to do it!
...slowly and progressively, of course :-)