Traditionally, companies would interview candidates face-to-face. But, with new interviewing technology and increased cost cutting, companies may moving more toward online interviews - at least for screening.
Companies like InterviewStream, Ovia, Hirevue and others offer alternatives to Skype where the candidates do not necessarily have to be online at the same time as interviewers, allowing even greater interviewing flexibility. Recruiters no longer have to incur travel costs or time away from the office.
As a job candidate, you should prepare yourself ahead of time for the possibility of a virtual interview. Virtual interviews have a different "atmosphere," so you must become comfortable with the online format before the interview day. Review Monster's Job Interviews: Tips for the Virtual Interview.