Virtual Idols are Idols who are virtual in nature, drawn, animated, etc… Virtual idols use drawn or animated images, but everything else is created by actual humans, like vocalists, dancers, music producers, etc. In recent years, these idols have performed live as well.

Examples include:

• 初音ミク (Hatsune Miku)
• ミス・モノクローム) (Miss Monochrome)
• The Idolm@ster Series (アイドルマスター)
• プリティーリズム(The Pretty Rhythm Series)
• Love Live!

See this article for further reading.