SaaS
In plain terms
Cloud model delivering ready-to-use software over the internet.
Going deeper
Software-as-a-Service provides fully-managed applications you access via browser, with the provider handling everything underneath. You just use the software.
Under the hood
No installation or maintenance for users; subscription-based, the most abstracted cloud tier.
Examples
Gmail, Salesforce, Slack, Dropbox, subscription web apps.
Think of it like...
Like streaming music instead of buying and maintaining your own equipment.