Several speedsters have taken the name of The Flash throughout DC history, from Jay Garrick to Barry Allen to Wally West, all these characters are consistently among the fastest beings in the DC universe. For nearly all of them, their powers have been kick-started during some ill-fated accident, had speedster parents, or in the case of at least one long-forgotten Flash retcon, by higher beings. Regardless of who they are, though, they all have an inherent understanding and connection to what is know as the Speed Force.
The Speed Force is what grants most Speedsters their superhuman speed and abilities, as it helps them negate the laws of physics that would otherwise limit their actual speed. The Speed Force also acts as a visible representation of every moment in time --- known as the Timestream --- which can be accessed by those who have made contact with the Speed Force.
Barry Allen is one such speedster who not only has accessed the Speed Force, but actively contains it. He has merged with the Speed Force itself, and it is through this energy field that he is granted his superhuman abilities, as it has affected nearly every major aspect of his anatomy. Other speedsters such as Johnny Quick, and Max Mercury, have also merged with the Speed Force, utilizing it to fuel their abilities as well. And yet others have proven more powerful than the Speed Force as a whole. Following the Infinite Crisis comic event, the Speed Force was seemingly destroyed, with every speedster losing their abilities... except for one.
Jay Garrick, the original Flash, managed to maintain his super speed despite the destruction of the Speed Force. While he can access and act as a conduit for the Speed Force, it is not necessary for him to use his superhuman abilities. While his original origin cites the inhalation of hard water (later, heavy water) as the source of his powers, it was later revealed that the inhalation actually activated a latent metagene in the hero. While technically a speedster, Garrick is a metahuman whose speed is derived from his genetic makeup.
In the pages of The Flash: The Fastest Man Alive #1, Garrick remains the only active speedster. While trying to stop a runaway truck of explosives, he hits a plateau, remarking to himself, "Out of breath...not getting slower...but not getting faster either. There's no Speed Force to tab...tougher to keep up." Garrick's speed comes from pure inertia, and despite being ranked the second-fastest man alive (second only to Barry Allen), he doesn't necessarily need to tap into the Speed Force for any reason other than to sustain his own energy, as well as gain an additional boost, although its effects have resulted in his slowed aging. As Jay Garrick proves, some speedsters don't need just the Speed Force to be one of the fastest people alive.