When you delete a project all teh contacts in that Project are removed from the project. If they are assigned to other projects there will be no change in those projects. If they are only assigned to the project you delete all the contacts will be placed in All Contacts without a project assignment. All history and data associated with any contacts in this project will be deleted. This includes notes, articles, links, payments, and any monitors.
To Delete a project:
go the settings icon and select Settings from the dropdown. Choose Manage Projects from the Settings menu.
find the project you want to delete.
click on the trash can icon to the left of the Project name.
Confirm that you want to delete this project. Please note again - Data associated with contacts in this project will be deleted (i.e., notes, article, links, payments, and monitors)