A Crossover is the placement of two or more otherwise discrete fictional characters, settings, or universes into the context of a single story.
Crossover blogs do this in various ways, a common way to do this is to "ponify" existing characters in order to mix the characters and themes from their universe into the existing ponyverse. Another way is to find similarities between a background pony and a pre-existing character or person and give that pony those traits and history. (Dr.Whooves and his fannon is a good example of this). The last way to crossover is to simply drop pre-existing characters from their universe strait into the ponyverse (or vice-versa) without any changes.
If a blog involves characters or themes that originate from a source other than the established MLP:FIM universe, it is considered a crossover.