Real image:
A real image is an image that you can project on a screen. The rays passes through the convex lens and bend towards a focus point. At a focus point where the rays come together an image is projected upside down, the real image. 
A real image can only be projected with a convex lens, because when a ray goes through a concave lens the rays will diverge and they don’t come together at a certain point.
There is only one situation when a convex lens forms no real image, that’s when there is an object between the focal point and the lens.

Virtual image:
A virtual image is an image that looks like an object that really exists, but it does not. Rays don’t meet so there is no image formed.