An Erection occurs when the arteries supply blood to your penis during physical or mental sexual arousal. Erectile Dysfunction (ED), also known as impotence, is the inability to achieve and then maintain that erection during sex. It usually affects men over the age group of 70.
Attaining an erection begins in the brain and then hormones, muscles, nerves, and blood vessels must work together to achieve and maintain that erection. ED occurs if there are any abnormalities with any of the system that work together to get an erection.
This condition is very common and most men would have experienced it occasionally. Only when this issue becomes persistent and when you cannot have satisfactory sex for over a period of time, say for a few weeks, then it can be a concern.