Mechanical Turk is an odd service. It’s called an “on-demand workforce” or peopleware. For large tasks that need to be automated but also require human intelligence, Mechanical Turk is the tool.
One of the examples Amazon uses is if you have 1,000,000 (one million) images that need to be tagged and categorized, you can use Mechanical Turk to “hire” 10,000 employees. You get to pick what you will pay and only those “turks” who want the work will sign up.
Amazon picks up 10% (additive) to whatever you pay someone.
FWS is the Amazon Fulfillment Service. This is another odd web service. Instead of using humans, this service has a warehousing component.
The short story is that Amazon has a world class fulfillment capability (i.e. ordering, packing and shipping products). FWS allows a business to tap into that by staging inventory in Amazon fulfillment centers.
Customers can order your products via Amazon.com or any other online or retail sales channel and your products will be packed and shipped from that center.
Actually storing your product at Amazon is called Fulfillment by Amazon and you pay to store your inventory. FWS is the web interface into the fulfillment center to allow you to programmatically send fulfillment requests from your web site or another merchant.
FWS (the API to make the requests) is free. FBA (the actual inventory part of it) is charged by the cubic foot of storage and how it is packed and shipped.