As a simple indication, Sinatra allows you to create an entire site by editing one (logically structured) file if you want.
Rails and Django make a few more assumptions about what you're going to do (eg creating a directory structure and giving you easy shortcuts to create forms to edit pages) that are pretty useful when you're building a webapp and want to create multiple reusable parts, but may be overkill for a simple HotorNot type page.
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Pardon my ignorance, but Rails and Django seem pretty lightweight as they are... How are sinatra etc lighter (and thus easier to use somehow)?
As a simple indication, Sinatra allows you to create an entire site by editing one (logically structured) file if you want.
Rails and Django make a few more assumptions about what you're going to do (eg creating a directory structure and giving you easy shortcuts to create forms to edit pages) that are pretty useful when you're building a webapp and want to create multiple reusable parts, but may be overkill for a simple HotorNot type page.