Open Project opens a closed project that is already in the current workspace. It is only useful if you actually close projects – personally I always leave them all open.
Import Existing Project imports a project from some other workspace and adds it to the current workspace. You can choose to copy everything in the project in to the workspace which makes it completely independent of the original project location, or you can leave the project pointing to original location.
Some versions of Eclipse also have a ‘Open Projects from File System…’ menu item which is variant of the Import Existing Projects but supports finding projects in a directory or archive (jar or zip). This will try and guess the project type even without the
.project file that identifies an Eclipse project and is required for Import Existing Projects.
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