ki is the internal energy which is inside of every living thing. One exception are Androids. This Ki is measured by power, is detectable with either concentrating your mind to read somebody elses power, or by a scouter which is used by the Saiyans and Freeza's henchman.Examples of Ki Usage:
Ki attacks: Attacks that are brought out internally which form small balls of blast which can do little to severe damage to the opponent. examples: Kame Hame Ha, Kikou Ha, Big Bang, ext.
Ki Detection: Ki Detection is a very hard move to accomplish because it involves a lot of concentration and patience. Everybody/thing has a Ki, and their signature Ki helps the people know who is coming, from miles away. Most of the characters in Dragonball/Z/GT can sense Ki mentally others can find out peoples Ki level by a divice called a scouter. Androids are the only things that DO NOT have Ki energy, that is why they could not find the Androids.
Flying: To fly, you need to concentrate your Ki very patiently, and use your Ki wisely,so you do not get tired really quickly.