Once baled, hay and straw can look similar, but they are very two different products.
What's the difference between hay and straw?
Hay is a crop, made from cut, dried grass, which is fed to farm animals and horses. Hay is not suitable for bedding.
Straw is the dried stalks of cereals like oats, barley, rye or wheat - the by-product, sometimes considered the waste part of the crop and has been used as a bedding for livestock and horses for centuries. It can be used as feed as well, although it is considered nutritionally poor.
These days straw bales can also be used for insulation in houses.
By growing a crop and baling the straw, farmers can make money from both cereals grown and from the waste stalks.
Compared to other products, like shavings and paper, straw is a cheap, easy-to-manage bedding option for horses and livestock but it can be dusty and cause problems in horses that have dust allergies.
In terms of scent, appearance and texture, hay smells sweet and is browner in colour, as it's made from grass. It's also softer than straw, which can be quite course and more yellow/golden in hue.
It is important that both straw and hay are harvested when the the weather is warm and allowed to dry thoroughly before baled in order to avoid mould spores developing. It is also crucial that both are stored in a dry location.
It's also important to note that grass clippings from the mower are not a form of hay and should NOT be fed to animals - so never throw them into a field nearby thinking you are giving the grazing horses a nice treat. Here The British Horse Society explain the dangers of grass clippings