Leaf curl is a general term for the curling of plant leaves, caused by pest, disease or chemical means. Peach leaf curl is a very common example - with peach leaves becoming curled and distorted due to fungal disease or sometimes pest attacks. In this case, prevention is always better than 'trying to' cure! Spraying with a copper fungicide prior to leaf break helps.
Peach and Prunus Tree leaf curl is normally accompanied with blistering of the leaf as well.
Potatoes are also prone to leaf roll due to virus attack. Aphids (greenfly, blackfly), can also cause leaf curl, (e.g. plum leaf curling aphid). Simply an infestation of aphids on the underside of a leaf can cause the leaf to curl up.
When aphids or other insects are the cause, then it is normally quite apparent. There are also a few caterpillars that curl the leaves around themselves, some even sewing the edges to do so! All forms of leaf curl caused by insects can be cured simply by spraying with an insecticide.
Leaf scorch however - identified by a typical 'scorched' appearance - can indicate a plant mineral deficiency, or a reaction to chemical sprays or even excessive sunlight in an un-shaded greenhouse.
Some leaves curl up naturally - better described as 'rolling' up than curling. Rhododendrons in particular will roll their leaves up in severe drought in order to minimise moisture loss via transpiration.