Both types of homes are built in a factory setting, but modular homes are usually customizable, are placed on top of a basement or crawl space, and often are indistinguishable from homes built on-location. On the other hand, manufactured homes are typically delivered as a single or double-wide format with a standardized layout, and do not require a permanent foundation (meaning they can be moved). Modular homes are not built to HUD construction code, cannot be moved from their foundation, and may be legally considered a traditional single-family home.
In general, all modular homes are manufactured homes, but not all manufactured homes are modular homes.