I'll give the answers in points with the corresponding question.
- A DB instance is a DB server. To put it simply, when you use RDS to launch a Database instance, you will be given a running DB server which you can use to create databases and store your data in. Now coming to the billing perspective, here 1 DB instance = 1 DB server which is running. Also, the charges for storing data, data transfer or taking backups are not included.
- To calculate monthly usage or hourly usage, you have an option in the Simple Monthly calculator:
- If your DB is already running, then you can upload data to it. Whenever your DB is running you will get billed according to the type of instance you have chosen. If you want your DB running all the time, then you should find a way to make it cheap with some best practices. If only for a few hours, then you will be charged relatively low and you will only get the bill once a month, so the charges are calculated hours per month basis.
As you are new to AWS, I would suggest you to take a look at this informative AWS Tutorial and also if you are looking to become an expert in the same, check out the AWS Training page.
For more information about Amazon RDS, check out the Amazon RDS tutorial page.