PowerApps is a tool designed for creating SharePoint Forms and Mobile apps.
Flow is a tool that automates business process.
A little bit more...
So what else?
PowerApps and flow add a new perspective to using Office 365 and the cloud in general. It is no longer necessary to develop large and complex systems that require huge resources. Using SharePoint brings your process, information and document control, PowerApps adds the controls and Flow add the automation.