Cross pollination (also called 'allogamy') requires the delivery of pollen from one plant to a separate individual plant.
This is in contrast to
which requires only the transference of pollen from stamens to carpels
within the same flower (or to flowers on the same plant specimen).
Plants that reproduce through allogamy are usually reliant on wind or other moving creatures.
In such cases, they will have developed specific features to attract
insects, or in some cases, bats of birds (as with the humming bird –
known to pollinate flowers in some countries, whilst it feeds on
In such cases, these plants are said to have developed ‘pollination syndromes’.
You can find more about how flowers attract pollinators (actually ‘vectors’) on my link here.
Additionally, they may have developed features within themselves to further aid cross pollination.
For a variety of reasons, humans may proactively cross pollinate plants via artificial means, for example, by hand, as a means of cultivating new varieties of plants.
Read more about plant pollination, including different types of pollination (such as wind and insect).