Canva is a drag and drop builder for designing logos, blog banners, website photos, flyers, posters, presentations and much more. There are millions of images to take inspiration from and you can even drag and drop them or upload your own images, combine them, change background and colors to make a desired design. You can simply see Canva as an online and limited functionality image editor tools like GIMP, Illiustrator or Adope Photoshop. The main advantage of Canva is the simplictiy with which you can use it and even non-designers can become designers with Canva. However, lately they have made a lot of their images paid and hence it is difficult to use Canva for free anymore.
Yes, Canva is completely free to use. The pre-designed images. logos or templates inside Canva has some pricing like $1 or similar which you need to pay for if you want to use them in your designs. You can just upload any image, use their drag and drop builder to combine them, make a pretty background (mostly free) and download it for free. Canva is better for newbies who do not have much experience in designing. The learning curve for Canva is much smaller than Photoshop and hence you can become a designer from non-designer in a day. However, the complexity of Photoshop is much higher than Canva.
Canva can be used online on any of the browsers and hence pratically anyone on the internet can access Canva from their desktops and from anywhere.
Canva has a few different ways to earn money like they have a subscription model where users pay like $12.95 per month to use some of the functionlities like making image background transparent or automatic resizing. Also, they have lots of custom designed images and templates which are watermarked and you can only use them in your designs after removing watermarks when you pay for them. They are priced really low like $1 per image and with subscriptions they even costs lower.
You can not sell Canva templates and designs directly but you can create your own designs using their platform which you can sell outside as 3rd party.