This question is asked in the majority of interviews so you should always be prepared for it. With that said it can be one of the most difficult questions to answer particularly for those who aren’t used to talking about themselves. But if you’ve prepared an answer beforehand then it makes the task easier because you won’t have to think on your feet. Bear in mind that the interviewer is assessing how well you could potentially fit into the team and culture of the organisation so make sure your answer is relevant. A good approach to this question is to list 10 words that describe you and see if there’s any correlation with the job description and person spec (if you finds there’s little or no relevance then this might not be the job for you). The job might require someone who is able to work under pressure in which case you should say you’re calm under pressure and able to deliver to deadlines. Give a real life example to punctuate your example. You could even talk about your weaknesses, but make sure that they are 'positive' weaknesses. For instance you can say that you are impatient with team members who cannot carry their own weight, or who cannot contribute sufficiently.
Preparation and practice is everything. Remember, if you hesitate about yourself, your interviewers will doubt whether you fit their bill of requirements. After all, if you don't know about you, who does?