Now I Get It: The Stormy Daniels-Trump saga: how did we get here?

On Tuesday, porn star Stormy Daniels, whose legal name is Stephanie Clifford, filed a lawsuit against President Trump to void a nondisclosure agreement that she alleges he never signed - and she did - which would have prevented her from discussing an alleged sexual affair with Trump. As the Stormy Daniels-Trump saga continues, how did we get here in the first place?