Democracy is a concept that manifests itself in any number of ways, all having in common that the people freely, at least theoretically, elect leaders who are intended to run the society on behalf of the people. However, even with a constitutional democracy, which is meant to protect the people from the vagaries of government, there are limitations. Democracy might very well be the best theory of government mankind has created, but it is by no means perfect.
First, as a general principle, in a democracy, the majority rules. This implies that at any given moment, there is a substantial minority whose needs are not being represented or met. This is clearly the case in the United States today, where, depending upon the issue, the American people are voicing minority opinions in numbers very close to the numbers for majority opinions. I am not recommending dictatorship as a superior form, but it is true, nevertheless, that there tends to be a certain equality of repression.
Second, in most democracies, leaders are elected for a defined term, and this implies that leadership can and often does change every term. This, to some degree, provides a lack of continuity and even instability in government, as opposed to a monarchy, which does, for better or worse, provide some continuity. In many democracies, civil servants are expected to provide stability in government services, but this has its limits, as each time someone new is elected, the agenda changes and the priorities from the top do have a profound effect upon the bureaucracy.
Third, democracies can too easily devolve into oligarchies, where a handful of wealthy people or entities can manipulate courts, legislatures, presidents, and voters into doing what is best for the oligarchs' interests. In spite of its having a constitution, at least on paper, this is happening in Russia today, and in spite of the United States Constitution, with all its checks and balances and guarantees, this appears to be the direction in which the United States is going, too.
Fourth, there is never any guarantee that people will vote in their best interests or make good judgements about candidates. A democracy can turn into a dictatorship fairly easily, since once someone is in office, he or she can suspend rights, instate martial law, or resort to some other means of seizing and maintaining control. The president of Burundi, for example, a country with a two-term limit, has insisted upon his entitlement to a third term. Bloomberg, the mayor of New York City, manipulated to do the same, and while he might not have been exactly what we think of as a dictator, there are those who are of the opinion that he wielded his power in a distinctly undemocratic way, the benevolent despot of the city.
Finally, depending upon how a democracy is constituted, it can lead to gridlock, where no one party is able to implement policy because of the distribution of power between the two parties. The United States has been experiencing this for some time now, first with a Democratic Senate and a Republican House, and more recently, with both being Republican but with a Democratic president who has the power of veto. In a democracy with multiple parties, this can happen as well, but in those cases, coalition governments are formed, compromises are made, and usually, something can get accomplished.
There are no doubt other limits on democracy, but it is, in my opinion, the best form of government there is. Entire libraries could be filled with commentary on the limitations and drawbacks of all other forms of government.