Look-up: a procedure involving a specific obituary, book page or chapter, or other document that can be handled in a short time
Research Request: asking that someone analyze, survey, organize, or otherwise work with genealogical materials
The difference is the amount of time and skill involved. An obituary doesn’t need to be analyzed to compare the deceased to his or her great-grandmother. A land record, however, doesn’t necessarily involve family relations, but does need analysis with other documents to establish relationships.
Look-ups can be, and usually are, handled by volunteers or library and archives employees who work with the records as part of the jobs. Research requests, because of the time involved are generally best handled by skilled or professional genealogists who make the time for such work.
Some genealogists like to post look-up requests on boards like Random Acts of Genealogical Kindness or FindaGrave. A few of these requests are asking for more information than what the volunteers are willing to find or capable of providing. Sites where genealogy researchers can find others more willing and able to do the in-depth research are eXpertGenealogy, GenealogyFreelancers, and GenLighten.
The level of skills presented by the researchers who list their services on these sites varies, as does their areas of expertise. Many professional genealogists give satisfactory service and their skills match those of certified genealogy researchers. A middle-ground of professionals who may or may not be certified, but are a group, can be found at the Association of Professional Genealogists’ site. Certified researchers who maintain listings can be found on Board for Certification of Genealogists’ site and the International Commission for the Accreditation of Professional Genealogists’ site.
Disclaimer: I maintain professional listings on eXpertGenealogy, GenealogyFreelancers, and GenLighten.