CRM / Dynamics 365 contains the concept of solutions. This means, everybody can build a solution in CRM.
What are solutions?
Solutions are like adding multiple documents to a zip folder to send them via mail to your friends. CRM also uses this concept, but instead of documents, you can add CRM-Items to this solution. For example a field of an entity, a form or view or the whole entity itself with all its dependencies. You can also add security roles, web-resources and workflows.
Why using solutions?
Solutions are used, to export and transfer items from one CRM to another instance. You can use it to develop some new features in your sandbox environment and after finishing your development, you build a solution and transfer the whole feature at once to your productive environment.
What is the difference between managed and unmanaged solutions?
If you export your feature with a managed solution, you can install this feature on your productive environment. But if you uninstall the managed solution, all fields and forms and web resources (= all customizations) you added to the system with this solution will be also deleted. ALSO the data will be deleted!!
The advantage of a managed solution is, that no user can remove components or change the solution.
Unmanaged solutions are packages, which only transfer the customizations from one environment to another. After installing an unmanaged solution, the customizations are written to the system like you have done them manually. This means after uninstalling an unmanaged solution, nothing happens. All customizations are still in the system. This is helpful to transfer you customizations but may offer problems, if you would like to roll back to the state before you installed the solution.