What limitations there are on a democracy depend upon what democracy is being considered. Democracy can take many forms, and so can the limitations. The democracy with which I am most familiar is that of the United States.
The United States is a constitutional democracy, meaning that the government is limited by the United States Constitution and its amendments. This includes the limitations created by the checks and balances amongst the three branches of government and the limitations referenced above, which protect minority opinions. So, for example, the laws that our elected representatives pass in Congress, even if signed by the president into law, can be overturned by the United States Supreme Court, in spite of the fact that the court comprises men and women whom we did not elect. Similarly, the president, whom we do elect, albeit indirectly, can propose an appointment that the Senate, which we also elect, may not approve. The careful balance of power that our Constitution sets up is meant to be sure that no one branch of government holds too much power, a kind of democratic limitation. Many of the rights afforded to us in the Bill of Rights and subsequent amendments are the kinds of limitations referred to above, assuring us that no matter whom we elect, there are rights that cannot be taken away from our government, for example, freedom of speech, freedom of religion, freedom of assembly, and freedom of the press. These are just a few of the rights we have from the Constitution.
Contrast this with a democracy that is not a constitutional democracy or a democracy that lacks a balance of power in its various branches of government or even a democracy in which there is only one branch of government, an executive branch. In cases such as these, the people might freely elect someone, which is the democratic way, but the people might choose someone who turns out to be a dictator. Without a constitution to protect the people's rights, there is nothing to stop this from happening. Without a constitution, there is less likelihood that a democracy can thrive and a greater likelihood of a violent transition of power, since an unconstitutional democracy can and does lead to people making bad choices from which they have no protection at all.