She has several choices. But to answer the difference, an oust in British politics requires the lack of support from key members of the government that the Prime Minister formed with approval from the Queen, as per the (misleadingly named) Unwritten Constitution.
So if you just can't form a cabinet, you can't form a government, and the Queen says someone else will have to be Prime Minister. That's an ousting. That's what happened to Thatcher after the Poll Tax; the people didn't really reject it per se, but the ministers just refused to work with her in forming a government anymore.
May still has a government. They're willing to work with her, but they have caveats and desires of the Brexit negotiations that May can't deliver because the EU consider the negotiations done. So, rather than waste more of her life slogging through the leadership work, she's had enough and she's done with the job. That's not an ouster. She could easily stick around if she wanted to. I wouldn't recommend it, but she could.