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When creating my first site collection it asks me to choose a template... but I have no idea (other than the little description) what the templates look like or how they behave, provide in terms of features, etc.
Is there an official site from Microsoft where I can demo and view these templates?
You can change your site collection template again after the site collection was created in the Site Settings > Page layouts and site templates, then you can test every template and choose the one who you liked more.
There isn't a site that fully demo's each OOB site template that I am aware of. You can do what Thales mentioned and create each of them to really see. If you want some documentation here are two posts that talk through the general details of each.
What kinds of sites can I create in SharePoint and Office 365?
