Is there a User limit?

Sites created with any of the plans do not have any user limit. However, these sites are limited by CPU hours per day. This means if there are a lot of users using the site, the site may slow down if the CPU usage limit is reached. For a detailed explanation read this blog post on Pricing.

What is the difference between Database and Disk space?

When you create data in your site, for e.g., Sales Invoices it will consume database space. When you upload files and images, it will consume disk space.