A virtual english keyboard is a keyboard that is only operated via software, i.e. there is no hardware equivalent. This is especially useful if you are in another country and want to write to your family from an internet cafe, for example, but the keyboard layout is different from what you are used to: as an English user you are used to the QWERTY layout, but in a French internet cafe there are only hardware keyboards with the AZERTY layout. Exactly in this case you call a virtual keyboard, either offered by the operating system or via a website like Lurki.com.
A virtual keyboard usually offers all the keys needed, which are selected with the mouse. This is much slower to type, but is usually sufficient to write a short text.