Hairline cracks
Fine, shallow cracks across the surface (often called crazing) are usually caused by the render drying too quickly or a mix that was too strong. They are mostly cosmetic. A flexible masonry paint will often cover them.
Cracks at corners, windows and doors
Cracks running from the corners of openings usually mean the wall is moving slightly or there were no beads or movement joints. Filling them only works if the render around them is still firmly stuck to the wall.
Blown render
Tap the wall around the crack. A hollow sound means the render has come away from the wall behind it. Water gets in behind blown render and freezes, which makes it worse each winter. Blown areas need cutting out and re-rendering.
Long cracks and bulging
Wide cracks, cracks that keep growing, or render that bulges out can point to a problem with the wall itself. Get it looked at before any repair, because re-rendering over a moving wall won't last.
Patch or re-render?
If the render is sound and the cracks are small, patching and painting is fine. If large areas sound hollow, re-rendering the whole elevation is usually better value than repeated patching, and the finish will match.