Considering how many characters Street Fighter IV has and how little time the game spends on explaining anything, you might be a bit confused by the storyline. You could read Wikipedia but why not just look at a colorful chart?

The plot diagram posted at Capcom Unity blog gives brief sketches of all the characters' relationships to one another. Dhalsim was investigating the Shadaloo Intimidation Network? I had no idea the guy even had a job.

Anyway, the chart looks a lot more complicated than it actually is. Would've been a bit more helpful if it included headshots of every character along with their name but what can you do?