First interview is normally by a HR officer who assess your general skills, personality among hundreds of other interviewees.
If you passes the first test (you are more qualifies than the 99 other people who came for interview), you will be called to meet your higher authority, such as your head of department or supervisor. If he/she likes your personality, you will get the job.
Sometimes second interview is called also because they had trimmed the applicants down to a few selected ones and the company would like to have a second chance to see which of the selected ones are more suitable for the job.