Theory that in markets displaying the Network Effect, early success translates into more and more success (momentum), resulting in significant marketshare concentration.
Examples: VCRs (VHS vs Beta), computer operating systems (Ms Windows vs everything else), etc.
In some cases this can be reinforced by Lock In properties - as long as the format for Ms Office keep changing, it's impossible for 3rd party applications to maintain file compatibility, therefore if you're going to have to exchange files with an Ms Office user (which is a near-certainty given their existing market share), then you have to buy Ms Office yourself.
This can lead to a Winner Take All market structure.
Some also apply it to Consum Er Brand Ing markets: where being the "biggest" (First Mover Advantage) leads to more name recognition, leading to a bigger share of future market growth, etc. - this is a more dubious argument.
Sometimes it's due to a Market Distortion.