When you graduate, you should be able to write down three or four words or short phrases that describe you and mostly nobody else. These words may change over time. When I graduated, I might have described myself as "payments, privacy, revokable". Now, I might instead say "phishing, mobile malware, user experiments". It is your goal to guide your research in a direction that allows you to find these words. Think of it like this: if your description is the same as hundreds of others out there, you will have a much tougher time getting what you want than if you are unique (and do good and relevant work.) Finding this specialty is not automatic or trivial, but it helps to keep in mind what would be a meaningful target.