Hi, Sarah. If you want to upload your whole family tree from a genealogical program on your computer or from a different website, you convert the information into a GEDCOM. It's kind of like an extremely compressed version of your family tree information. Our help page on GEDCOMS says the following:
"GEDCOM (which stands for GEnealogical Data COMmunication) is a standard file format for transferring family tree information."
No one has to use a GEDCOM on WikiTree. Manually creating profiles one at a time gives you more control in that you can check the privacy level and verify information for each profile as you build your family tree. Often, people have spent many years building their family trees and have thousands of names. They don't have the time or the inclination to repeat all that information by hand so they use a GEDCOM to upload the data all at once.
Family tree information you add to genealogical software on your computer can be converted to one large or many small GEDCOMS. Some of us use a combination of both manual data entry and uploading smaller GEDCOMS.
If you'd like to read more about GEDCOMS, following is a link to our GEDCOM FAQ: http://www.wikitree.com/wiki/GEDCOM_FAQ