I paid £20ish for it in a Steam sale and then found the code so that we didn't have to keep opening Steam to use it.
You can use it across different platforms, it syncs using dropbox.
It takes a while to set up, but as long as you maintain it as you spend (or as money comes out of your account) then it doesn't take time to maintain, DP and I update ours anywhere between once a week to once a month and it takes from ten minutes to an hour depending on how much spending we've done, how long it's been, etc.
As Lila said, you categorise all of your spending, and it relates to a budget plan. The idea is that you budget for every penny that comes your way, so that it doesn't get left behind, even if it's in an 'overflow' (which we use to top up our categories if they run low).
You can download bank statements and upload them automatically to YNAB, or you can manually input your spending into it. I prefer that method tbh, as it makes me aware of when/where I am spending, but the uploading is much easier!