Although some cultivars are self fertile, even these fruit better if they are cross pollinated. The majority are not, and need pollination from a different cultivar of the same fruit type that flowers at the same time, which needs to be within 18 metres away. For apples, the alternative is to plant a crab apple, which will pollinate just about everything as their flowering is so extended and prolific, and if you don't use the fruit the birds will enjoy it.
We have adopted the RHS pollination group numbers, which divide apple, pear, plum, and cherry cultivars into 7 flowering periods. Pollination group 1 is the earliest to flower, and trees in this group will pollinate those in the same group or group 2. Trees in group 2 will pollinate those in groups 1, 2, and 3, and those in group 3 will pollinate trees in groups 2,3,and 4 - and so on. We have indicated which cultivars are triploid - i.e. having sterile pollen and needing at least two other non-triploid cultivars.
Pollination can be dramatically improved by having honey bees nearby, encouraging wild pollinating insects - particularly bumble bees - and planting good sheltering hedges to reduce wind speed.