Yeah you can think of rails as requiring a database. You can pretty easily substitute whatever you want at the database level (sqlite3, mysql, postgresql, mongodb, even redis!), but it's the convention that you'll need to store and retrieve data in your webapp. If you don't need to do this (edge case for most of us), then try a different framework like sinatra that isn't accompanied by all the database bloat.

If you're planning on using a DB eventually but don't want to deal with migrations etc. for now, then start with a NoSQL DB like MongoDB (you don't even need to describe your fields, it can be completely dynamic). You can always sub in an RDBMS later on if your design calls for it.