There is a need to include certain files when commiting to repositories. For example, if your rails application creates a development.log file on your local environment, you certainly don't need to commit that.

There is also the need to provide global exclusions for files, for example MacOS .DS_Store files or Windows Thumbs.db, hence we need a global exclusion list

In order to define the global exclusion list, we can run

$ git config --global core.excludesfile ~/.gitignore_global

This will create a file .gitignore_global under our home directory, where we can start adding patterns that may be excluded

➜  ~  vim ~/.gitignore_global
.DS_Store
*.orig

Alternatively you can ignore an entire folder, as an example you may want to ingore a log folder.

➜  ~  vim ~/.gitignore_global
.DS_Store
*.orig
logs/

As you can see we've excluded MacOS .DS_Store files and *.orig files which are the backup files that git creates after merging

The same functionality works within repositories themselves, only difference is that you need to name the file .gitignore

On the other hand, since for example empty directories are omitted, in order to keep a directory (eg. the directory structure for the logs directory, but not the log files themselves), we may add an empty .gitkeep file.