For example, what should I expect from a "Senior Ruby Developer" that's different from what I should expect from a "Junior Ruby Developer" or just a plain old "Ruby Developer?" Are the distinctions standardized? Like with a test or a certificate or something?
I will keep it simple. A junior developer can write an app. A senior developer can write the app and also the backend service. An architect level developer can not only do these things he/she can figure out performance, scalability and maintainability issues.
An architect level developer will have the emotional maturity to pick technologies that will solve your problem in the best possible way without bias to any favorite technology stacks they use.
a junior developer can implement to the the design/direction that you give them. It's possible but not likely that they will think outside the parameters that you give them (e.g. security, error handling, code reusability, etc)
a senior developer can take a problem than you give them and design a solution that addresses that problem. The senior developer will assess parameters like security, error handling, code reusability, negative path test, etc).
The difference is in the years of experience. A Senior Developer should be able to complete tasks more quickly and with fewer bugs than a Junior Developer, because they have done similar work before.
If you have a mission critical app with little room for error, it will pay dividends to hire the senior developer. If you have more wiggle room, maybe it makes sense to pay the Junior Developer less to learn on the job and take a bit longer to get the job done.