Besides controlling the amount of light your camera receives, using longer shutter speeds blurs motion. For example, making a waterfall look very smooth and feathery. A fast shutter speed freezes action, such as someone kicking a soccer ball. For more on how to manually expose a scene (controlling shutter, aperture, ISO) check out this 14 minute tutorial on manually controlling exposure.
If you really want to get into the details of photography, I also wholeheartedly recommend Tony Northrup's book, Stunning Digital Photography. Starting at $10 for the ebook, available on his website or Amazon. It walks through technical and artistic aspects of photography; containing chapters on specific genres such as portrait, landscape, and so on. Many of the videos from the book are available for free on his YouTube channel, such as the one I posted above.
As far as street photography goes, I'm not hugely into it, but Henri Cartier-Bresson is considered a pioneer of the genere.
Any of those would be great starter cameras. I would steer you towards the Nikon as their camera sensors are a bit better than the Canon ones.
If you go for the D3300, check out this 1 hour tutorial on using it. That channel also has tutorials on Canon if you go that route.
If you really want to get into the details of photography, I also wholeheartedly recommend Tony Northrup's book, Stunning Digital Photography. Starting at $10 for the ebook, available on his website or Amazon. It walks through technical and artistic aspects of photography; containing chapters on specific genres such as portrait, landscape, macro, and so on. Many of the videos from the book are available for free on his YouTube channel, such as the one I posted above.
First I am impressed with all the great answers everyone gave you! I am going to recommend a book under ten dollars on Amazon that will also give you 20 hours of up-to-date video instructions. Check out How to Create Stunning Digital Photography, Kindle Edition. Buy the Kindle version you can open it on your iPhone, iPod, or laptop. It is a simply awesome book for any photographer with questions: https://www.amazon.com/How-Create-Stunning-Digital-Photography-ebook/dp/B006KY2VZ2