Copyleft licenses are a type of free and open source software license that ensures the continued openness and freedom of the software and its derivatives. They require that any modifications or derivative works of the software be distributed under the same terms as the original software. This means that the source code must be made available and anyone who receives the software must have the freedom to modify, distribute, and use the code.

Here are some major copyleft licenses:

1. **GNU General Public License (GPL):** This is one of the most well-known copyleft licenses. It requires that any software or derivative work based on GPL-licensed code be distributed under the terms of the GPL, meaning the source code must be made available.

2. **GNU Lesser General Public License (LGPL):** This license is similar to the GPL but provides more permissive terms for software libraries. It allows developers to link LGPL-licensed libraries with proprietary software without requiring the proprietary software to be open source.

3. **Mozilla Public License (MPL):** The MPL is a weak copyleft license that allows combining MPL-licensed code with proprietary code under certain conditions. It requires that any modifications made to the MPL-licensed code be made available under the MPL.

4. **Eclipse Public License (EPL):** This copyleft license is specifically designed for software development in the Eclipse ecosystem. It allows the use of EPL-licensed code in proprietary software under certain conditions.

These are just a few examples of copyleft licenses. Each license has its own specific terms and conditions, so it's important to review and understand the terms of a particular license before using or distributing software under that license.
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