Amazon Web Services allows the creation of Virtual Machines in a Cloud environment. A user can create anything from a single VM running a web server, to a multi server system containing many Virtual Machines. Evert hing you create in AWS has a potential cost, and these items are (but not limited to):
Processor (vCPU) type and quantity
Amount of RAM
Amount and Type of Storage
For this overview, we are going to be using the Free Tier components, this means we can create an AWS virtual machine with components that cost nothing for the first 12 months. Actually, if you don't exceed your resources, you can play with the numbers, you actually get 750 hours per month on the t2Micro server. This means, you could run two servers for two weeks without incurring charge, but they would have to be down for half the month as well.
This example uses the free tier server, RAM and HDD options to give you a server that you can run for free, for 12 months. When I say run this is important, running a server might be free, but if you start to move a lot of data into or out of this server, you may also incur costs, so be careful (there is a built in cost explorer, you can set alerts for charges and you can see any charges very easily.)