A "Job" is similar to a Customer, it's sort of a sub-customer. You can use it to track transactions and contact information separately from other "Jobs" of the same Customer. It would be useful to track different projects for a Customer. For example, if you have Customer Joe Schmoe, you could have a Job for a project where you build an extension to their house, and a separate job for doing some work on their yard. This would allow you to have contact information for separate contractors, as well as to track transactions such as Invoices and Payments separately. Customer:Jobs are "Entities" and sync between Method and QuickBooks.
A "Work Order" is an order for products or services created internally to reflect a request on the part of a customer. Depending on the kind of product or service your business offers, your work order might express a wide variety of information: cost of a product, an hourly rate for service, discounts and taxes, and much more. Work Orders are based off of Activities since they typically involve a time and place that work will be done. For example, Employee Amy McName is to be at Joe Schmoe's house at 12 PM on Wednesday to plant a hedge (hypothetically part of the Job for Joe Schmoe's garden). Work Orders do not sync between Method and QuickBooks, they are Method-only.
A "Sales Order" can be defined as a company-generated record of what has been sold by your company, to whom, and when. The reason a sales order exists is to create an internal record of all the pertinent details associated with a sale that allows for internal auditing. When you create and fill out a sales order, you haven't actually recorded the sale - you've only recorded the information you need to make sure the order is fulfilled. The sale is recorded only after you create an invoice. Sales Orders are "Transactions" and sync between Method and QuickBooks, assuming that they're supported by your version of QuickBooks.
If you haven't yet, you might want to take a look at our Help Center, it contains documentation and video tutorials about Method and its features. You can find it here. If you have any other questions, don't hesitate to post here on our forums.
Hope this helps.