This is not really a summary, but it's the first thing that comes to mind when I think of Order of the Phoenix:
Seriously. Umbridge is freaking scary. I would rather play Quidditch with Voldemort using a dementor as referee (and while we're at it, let's assume the audience is nothing but Slytherins) than take one step into that woman's office.
For those of you who wanted a real summary, here is basically everything else that happens in the fifth book:
Harry is kind of an angry jerk to everyone.
And that's pretty much it.