Crickets and grasshoppers belong to the Orthoptera group which literally means 'Straight Winged'.  They feed using the specialised mandibles and primarily devour a menu of vegetarian fare, although some of the Crickets will give a nip of the flesh if troubled and perhaps lap up a few drops of resulting blood.
The life cycle of the orthopterans is an incomplete metamorphosis which involves only 3 stages - egg, nymph and adult.  Once hatched the nymphs moult several times and become fully mature adults which are ready to breed.  Numbers fluctuate from year to year primarily dependant on weather conditions and availability of food.  Some years 'swarms' can erupt whereupon food crops can be affected.
The are several difference between crickets and grasshoppers, the main ones being:-
Crickets tend to have long thread-like antennae whereas grasshoppers are thicker and shorter.
Crickets are usually more active in the late afternoon, evening and at night whilst Grasshoppers like being active in the day.
The auditory organs of crickets are positioned on the forelegs whilst those of grasshoppers are on the abdomen.
Crickets stridulate by rubbing the forewings together, grasshoppers rub the hind leg against the forewing.
The ovipositors of crickets tend to be long, grasshoppers usually short

Not all of these characteristics are a 100% rule of thumb but in the main can give indication as to the species observed.
In the British Isles there are 27 native species of Orthoptera (grasshoppers and crickets) and a number of non-native, naturalised species.  Due to climate change many species are expanding their range and it may be that these isles may become home to several different species.