You are really comparing chalk and cheese. WMS serves up raster tiles of data that have been created by a WMS server. This data can originally be vector or raster data and will have been drawn with style, but by the time it gets to your client it is just a raster image. WFS on the other hand, serves up vector data.
Good places to look are the OGC website which has the standards documents as well as overview documents for the various services. This would be a reference to put in your bibliography. MapServer and GeoServer have lots of documentation on the way each of them deals with WMS and WFS. And of course, there is Wikipedia which has short but generally accurate pages on both, but I suspect you would lose marks if you actually cited the site in your thesis. Best just use it to get an overview.
With something like these geospatial web services, it usually pays dividends if you actually install a server and play around with creating maps. There are plenty of tutorials out there, and there are lots of previous posts on stackexchange.
My undergrad thesis is on the use of web mapping for the archaeology industry, and I have a virtual machine (VBox running JeOS Ubuntu 10.10) that runs mapserver and tinyows that I can connect to like a "real" server, and test what I'm writing about.