An Eclipse project consists of a directory, project settings files and at least the contents of the directory. This means that only one Eclipse project can exist in any one directory. Eclipse projects build their sources to just one form of output (such as an executable, an .exe, or a dynamically loaded library, a .dll) which you control using the project's build configurations.
Projects have a type (such as JVM COBOL) and you create them using a wizard for the appropriate project type. The type of the project determines how the project node is decorated (what icon is used for it) and what view it is opened in inside the main Eclipse window - for example, the COBOL Explorer view for COBOL projects.