Django's CharField requires a max_length, and TextField displays a multi- line widget, but in Postgres there's no reason to add an arbitrary max thanks to the `varlena` storage format. So this is a TextField that displays as a single line instead of a multiline TextArea.
Unlimited-length character fields in Postgres perform the same as limited-length fields, and the Postgres manual suggests not arbitrarily limiting these fields. Unfortunately, Django does not provide a way to access unlimited-length character fields except using TextField, which is rendered differently in forms and in the admin, and has different connotations. LongCharField is a way to avoid putting arbitrary max_length values where they aren't required. It will only work with databases that allow VARCHAR with no numeric parameters, such as Postgres. MySQL won't work.