The main difference between brownfield and greenfield developments is that brownfield developments are built on land that has already been developed, while greenfield developments are built on undeveloped land. Brownfield developments are generally located in more central locations, have existing infrastructure and transportation links, and may offer larger potential financial incentives.
For example, replacing a dated office building with a shopping center or constructing an affordable housing community on the former site of a hospital would both be considered brownfield developments. Greenfield developments, on the other hand, are typically built on undeveloped land, such as a new housing development in a rural area.
For more information, please see this article on the HUD Loans blog.