A flowing world of mystery, silver, morphing hyper-dimensional gels. Speed Heaven, the source of my power. 
- —Wally West
History
The Speed Force is an energy field that grants all speedsters their power. Several speedsters have merged with it, including Barry Allen, Johnny Quick, and Max Mercury.
Scientists on the planet Savoth had a deep understanding of the Speed Force and there were many universities devoted to its study. One of these scientists, Gorflack, built a Speed Force cannon to defend his planet from a force of alien invaders. Instead of performing its intended purpose, it brought Jay Garrick, the first Flash, to Savoth. After Garrick helped drive away the invaders, Gorflack gave him a lightning-shaped piece of solidified Speed Force which he and his successors could use to transport to Savoth.[1]
While fighting Superboy-Prime, Wally was nearly swept into the Speed Force. Wanting to see his family one last time, he visited his wife Linda and twin children. However, Linda was holding the Speed Force fragment and the entire family was swept off to Savoth.[2] Bart Allen volunteered to absorb the entire Speed Force so that he would be fast enough to stop Prime.[3] After this crisis had been averted, Bart found that his new connection to the Speed Force threatened to kill him. He refused to use his powers for a year. It wasn't until Tina McGee and Valerie Perez of S.T.A.R. Labs were able to modify a Flash costume to contain his abilities that he was able to return to action as the fourth Flash.
Shortly thereafter, Inertia; who had lost his connection to the Speed Force when Bart absorbed it, was forced to use Velocity 9 to maintain his super-speed. Inertia built a machine in Los Angeles which was designed to transfer the Speed Force from Bart to himself. However, the machine also threatened to cause an explosion which would have resulted in the deaths of millions. Bart sacrificed himself distracting Inertia and a group of Rogues while Valerie shut it down.[4]
The deactivation of the machine caused a disruption in the Speed Force which not only resulted in Wally West and his family being transported back to Earth from their stay on Savoth, but also restored powers to all speedsters.[5]
The sole survivor of the alien invasion forces that had attacked Savoth decades prior was sent back to his home dimension in a Speed Force explosion which resulted in a rapid evolution of his species. His descendants targeted the Flashes' solidified fragment of Speed Force and invaded Keystone City to avenge their defeat on Savoth. Wally destroyed the fragment to send them back.[2]
Those with access to the Speed Force can use a large number of abilities, usually revolving around speed, including: Superhuman Speed, Superhuman Endurance, Increased Perceptions, Accelerated Healing, Decelerated Aging, Speed Force Aura, Supercharged Brain Activity, Vortex Creation, Phasing, Steal Speed, etc.
Notes
Different conduits throughout the multiverse excel in different Speed Force -related abilities, and it furthermore has differing effects on all of its users. Some examples being the original Flash, Jay Garrick's decelerated aging as opposed to Bart Allen's over-accelerated aging process. Others examples being how the Tornado Twins excel in using their speed to create high velocity whirlwinds that allow them to take flight, while Bart Allen has shown the ability to divide himself into multiple physical selves. Lady Flash, who is the keeper of the Speed Force in Earth-33, casts electrokinetic blasts that are capable of temporarily draining the movement of her target. And as the third Flash, Wally West learns how to manifest his costume at will, being created purely from the solidified Speed Force. Prior to the loss of his connection to the Speed Force, Jai West's super-speed could temporarily accelerate the growth of his muscles, giving the child super-strength. Notably, there is also the Negative Speed Force, which as the name suggests, negates and opposes the average Speed Force energy. Professor Zoom (and apparently Black Flash) are users of this super-speed that manifests as red electricity.