Dedicated Command Line Tools
The safe way to add or remove software repositories using the terminal is using the command add-apt-repository. For example:
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sudo add-apt-repository ppa:libreoffice/ppa
will add the libreoffice Fresh PPA. Using the -r option removes it again:
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sudo add-apt-repository -r ppa:libreoffice/ppa
This command is installed by default in Ubuntu.
Alternatively, to remove a ppa, you could install ppa-purge. Next to removing a PPA, that script will also automatically revert packages to their versions of the official Ubuntu software sources, or remove them if they are not available there. For example:
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sudo ppa-purge ppa:libreoffice/ppa
Manually changing /etc/apt/sources.list
You can also manually edit /etc/apt/sources.list or /etc/apt/sources.list.d to add repositories, but then you need to know the correct syntax. See man sources.list 5 to find information about the required format. /etc/apt/sources.list.d alows to add entries as separate files instead of as a section in /etc/apt/sources.list. Only files with the extension .list or .sources - which have different formats - are interpreted as individual entries. Other files are ignored. To remove a repository registered in this directory, delete its .list or .sources file.
Graphical tool: “Software & Sources”
Finally, also consider the graphical tool “Software & Updates”. With this tool, you can enable and disable official software sources, and add, enable or disable third party PPA’s using graphical dialogs. Also that tool is available by default in Ubuntu.