Third-Party programs are different kinds of tools to assist in various tasks. Usually this tools are used as alternatives or to complement in-game features because they allow for a more efficient and user-friendly experience.
The most popular kind of tools are game plugins, and tools that allow to record video sequences from demos.
Lawena Recording Tool
Lawena Recording Tool is a cross-platform tool coded in Java, originally developed by Montz and now maintained by Quantic, it provides a flexible user interface for the recording of image sequences from TF2. Lawena is considered the spiritual successor to the PLDX tool who was one of the first and the most popular tool used for recording tf2 videos before it was discontinued.
The tool allows through it's interface and settings the recording of clips from demos with ease, making it an essential tool for the tf2 competitive community. It has been used in numerous videos and many of the most famous fragmovies used this or a similar tool to record clips from demos.
P-REC (POV-Record) is a third party plugin created by Orange, the plugin is verified by Valve. P-REC is a useful plugin, it is used for easy and automatic demo recording in Team Fortress 2, it automatically starts recording demos when tournament mode is activated, but the settings allow for a great customization of the plugin according to the player needs. It also has a auto-screenshot feature that screenshots the final map score and the console status, this is particularly useful for European players due to how ETF2L system works for submitting results, which require map and status screens.
Valve has created its own demo system, located in the Advanced Options.