A short, permissive software license. Basically, you can do whatever you want as long as you include the original copyright and license notice in any copy of the software/source. There are many variations of this license in use.
The MIT License (MIT)
Copyright (c) <year> <copyright holders>
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
Describes the ability to use the software for commercial purposes.
Describes the ability to modify the software and create derivatives.
Describes the ability to distribute original or modified (derivative) works.
Describes the ability for you to grant/extend a license to the software.
Describes the ability to use/modify software freely without distributing it.
Describes the warranty and if the software/license owner can be charged for damages.
Describes whether the original copyright must be retained.
Including the full text of license in modified software.