My views below. Note that I have gone from being something of an advocate for Power Apps to now being quite critical of its evolution in terms of reliability, feature-set and licensing so please keep those biases in mind.
>>1) What exactly is Power Apps? How is it different from Power BI? I read something about Websites and Apps?
Power Apps is used to create small CRUD (create, read, update, delete) type applications that can run either in a browser or in an 'App Player' that can be downloaded from Play Store and the Apple Store (there is a player for Windows but it has long been abandoned and shouldn't be used).
They also have hundreds of 'connectors' for other services, such as email and the Office Users list, so you can do things like check who is the manager for a user, send an email, book a meeting in a calendar, etc. In my experience many of the connectors are not on a par with the equivalent API calls though, in terms of exposing the full, current set of capabilities.
2) Do you think Power Apps would be a much needed asset for any department working on big data?
No, they are not for data analysis or data presentation.
3) I noticed that Power Apps visuals can be used in Power BI are people here utilising it?
You can place a Power App inside Power BI as a visual, so if you need someone to be able to write a record, send an automated email, etc. based on what they see in the Power BI screen, you can do that. The Power App can be aware of the dataset in Power BI, so you could be viewing a record in a Power BI report and hit a button to update some underlying data or fire off an email with the details to the relevant person.
How are these websites different from ordinary websites and apps that people can make on other platforms?
It is generally easier to get started with Power Apps - the language is simpler (though more limited) and the design tools run in the web browser so there is nothing to install/configure on the development PC. The flip-side of the simplified language/set-up is that there is less code-reuse/encapsulation and maintenance and debugging can be challenging to say the least.
The infrastructure is part of Microsoft365 so there are no servers/services to configure or update but you cannot run them anywhere else and you cannot opt out of patches/updates, so it is not a good idea to use them for critical purposes (in case an update breaks them or changes behaviour or if you don't want to be tied to MS indefinitely).
You get some base capabilities included as part of most Micorosoft365 offerings but for serious Apps you will need 'premium connectors' which require additional licensing which is generally on a per-user basis (there are now some slightly more flexible licensing options). I find the strategy here odd to say the least - for example if I want to use my Microsoft Power App with my Microsoft SQL Server I have to pay an extra fee (for each app and each user) for MS to allow me to connect the two together.
