Heart Rate is a record of what your Heart Rate is at a moment in time. It is a count of how many times the heart beats in a minute. Heart Rate changes depending on the demands your body is putting on your heart. This is shown in the picture above.
On a graph of how heart rate changes with time (over an hour, a day or a night), there will be lots of ups and downs but this is NOT an ECG. For the home user, Heart Rate is more useful than ECG.
An ECG picks up information of how the heart beats. Are the beats regular or irregular, does it look like there might be a problem with how all the parts of the heart work together? ECG recorders can have auto-interpretation built in, but even for high-end hospital ECG recorders, this can only be considered as a “silent second opinion”. So while it is great to be able to record your own ECG like the Alivecor below, it is really for a doctor or Physiological Measurement Technologist to read it and interpret it properly.
For information on normal heart rate, see this link from the Mayo Clinic:
Most smart watches and wearables measure heart rate. Very few measure ECG. The picture below shows the ECG from the Kardia (also known as Alivecor) ECG recorder – one of the few smart phone ECG recorders.