RDS (Relational Database Service)
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What is it?
RDS provides database servers. These are used for storing, organising and querying data; on a super small website you could do this on the webserver, but in anything larger than that, you need a dedicated database server, or multiple database servers.
Why did we choose it?
Integration with other AWS services. It's not impossible to use another provider, but RDS is reasonably priced and works well, so there wasn't a reason to consider using another provider for it (unlike say, Amazon's CDN offering named CloudFront, which is overwhelmingly expensive).
What do we use it for?
For keeping all of Deserted Chateau's data. Things like usernames, passwords in encrypted form, paths to images and so forth are stored there. We do not keep sensitive information in our databases as it isn't needed for our purposes.