PowerApps is a tool designed for creating SharePoint Forms and Mobile apps.

PowerApps has the ability to directly interface with SharePoint content, allowing you to create rich, dynamic forms and apps that can be used to best leverage your SharePoint solution.

Mobile App
PowerApps also allows you to connect to any source and produce a bespoke mobile phone app, giving external users a simple way to interact with your data.

Other Stuff
PowerApps also has connectors for other Data locations, like on-premise SQL and even Excel!

Microsoft Flow

Flow is a tool that automates business process.

Automatic Approval Processes
Out of the Box, flow has an approval process that can be attached to many different tasks. For example, if submitting a form in a PowerApp, it can very easily be setup to run an approval process.

More complex tasks
Flow also allows you to get very granular, it can run some very complicated tasks as well. For example, you can put in choices, for example, if a yes/no control was set to yes, email someone, but if no, email someone else.

Other Stuff
Flow can also connect to external sources, like twitter, or even an external data source like Dropbox! You can set automatic tasks to something like 1500 external apps!

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.