To begin with, geology is the study of the earth. This includes rocks, minerals, and everything they make up, such as the earth's crust, fossils found in the earth's crust, and the results from the movement of the earth's crust, like volcanos and earthquakes. Geology also studies how water moves and changes the earth above and below the surface, and any other processes involved in how the earth functions.
Field geology is usually used to refer to learning and using geologic techniques out in the 'field' as opposed to work done in a laboratory. This could mean anything from surveying a piece of land, taking soil samples, preparing geologic maps and cross sections, finding fossils, and any number of other tasks done out in the field instead of in a lab or an office.
Generally, geology is the study of the Earth. There are, however, many sub-fields of geology such as fossils, rocks and minerals, volcanos, the land masses, etc. While there are many fields of geology, they all overlap in their studies. Examples would be how the mineral composition of a fossil could help determine how old it is or how the shifting of the tectonic plates (land masses) could cause a volcano to erupt.