A panel interview is a job interview conducted by more than one person. Generally, there is still only one interview, but they will have multiple people interviewing them. This often occurs in larger companies where more people want input on recruitment decisions.
It is most common to have three interviewers in a panel interview. These will often include the hiring manager, HR representative, and a colleague of the available position or another manager. Panel interviews are helpful, as the interviewers can compare notes on the different applicants and can offer feedback to the hiring manager, who will likely be the final decision maker.