A daypack is a smaller version of a rucksack, light enough to hold a few extra small items and usually used for shorter day hikes.
Most daypacks have a frame of some sort, which helps distribute the weight and keep the pack from sagging down too low on your back. Some have internal frames, while others have external frames. Daypacks also usually have some sort of padding on the back panel and/or straps to make them more comfortable to carry.
Some daypacks have a built-in rain cover, while others require you to purchase one separately. If you’ll be hiking in areas with a lot of rainfall, make sure to get a pack with a rain cover.
If you’re planning on doing some backpacking as well, you might want to get a daypack that’s big enough to double as your carry-on bag for your backpack.
Whatever your needs, there’s a daypack out there that’s perfect for you. So get out there and start exploring!