Having worked for an appraisal company for 4 years, I can tell you a bit about the appraisal process.
If they are doing a full appraisal, they will come out, look around the house, probably take measurements of the outside and inside (unless they have a reliable blueprint of the house, and even then they should measure to confirm). They will take lots of pics of the inside and outside of the home. How they assess the value is by comparing your home, area, etc. to other properties similar to yours that have sold recently. In the US there is MLS, Multiple Listing Service, that is used by realtors to list homes for sale and record sales. They will try to find the most similar comparables they can. These will be listed in the appraisal, with pics and information about the comparable (square footage, HVAC, amenities, location, upgrades), so you can see where they got the value that they did for your home.
Drive By appraisals are just what they say they are. They use the data from a previous appraisal and public records for the information on your house, and just drive by to see the overall condition of the home from the outside. From there the process is the same.