What is mirrorless camera?

Mirrorless cameras are very trendy right now. What is it? Mirrorless cameras are digital cameras, that are a mix of compact point-and-shoot cameras and single-lens reflex (SLR) cameras. Mirrorless cameras have compact body, like point-and-shoot cameras does. Mirrorless cameras also have interchangable lens, like SLR cameras. However, it doesn't have mirror-based optical viewfinder, like SLR cameras does. Instead, you are using the screen to frame your pictures. Seems like there are two different types of mirrorless cameras on the market, DSLR-like ones with big sensors and compact ones with pancake lenses. If you are planning to change from SLR camera to mirrorless camera, be aware that your old lenses may not go with the mirrorless camera! For example, Canon DSLR cameras use EF lenses, but the mirrorless Canon cameras use EF-M lenses.