A groin vault is produced by two barrel vaults. It is an architectural feature specific to arches. A groin vault occurs when two barrel vaults intersect at right angles. Because barrel vaults are essentially tunnels, this is a connected and whole vault. The definition provided is: "A vault produced by the intersection at right angles of two barrel (tunnel) vaults. Sometimes the arches of groin vaults may be pointed instead of round."
In contrast, a rib vault is more like a network or frame. It can look like ribs, hence "rib vault." The definition provided is: "a masonry vault with a relatively thin web and set within a framework of ribs."
The groin vault was introduced early, used in Roman architecture. The rib vault became popular as Gothic architecture was introduced.
A groin vault is a certain pattern in architecture where vaults meet together in a V. These are made," by building two barrel vaults, that cross each other, so that they make an X." This was commonly used in Romanesque churches.
A ribbed vault looks like a groin vault but has ribbed arches. The pictures below make it more easy to understand the difference.