A meadow (properly called a hay meadow) is a field in which trees are prevented from growing because of cutting for hay rather than grazing by animals. As a result, the plants that grow are grasses and other non-woody plants (herbs). Some herbs (like poppies) are annuals, while others (like daisies) are perennials. This is different from a natural grassland like a prairie where grasses and other herbs dominate because it is too dry for trees to grow.
Meadows are important because far more species can grow if sunshine can reach an area all summer. Under normal conditions trees would shade the area and just a few types of plant would thrive.
Traditionally hay was cut once a year, usually in later summer after the wildflowers had set seed. More recently, hay has been cut earlier, but this is not as good for meadows. As a result governments have given cash bonuses to farmers who only mow for hay after the seeds have been produced.
If an area of meadow were grazed by animals. they would eat the young shoots before they could even flower, so a meadow will only occur where there are no grazing animals - at least until after the seeds have set.
Meadows found by rivers in floodplains are often called water meadows.
A pasture is a field of grasses and herbs that is grazed by animals through the summer. It is mostly very leafy grasses and clover.