Can anyone explain to me the difference between content type and custom content type?
Custom content type can be used with two different meanings:
Any node type (bundle, or entity bundle, in Drupal 8 terminology) created from the user interface (admin/structure/types/add in Drupal 7 and Drupal 8) from users with the right permissions
Any node type implemented from third-party modules
What is not a custom content type is a content type created from a Drupal core module, or a Drupal core profile. For example, the Forum topic content type is created from the Forum module, while the Article content type is created from the Standard profile.
For the rest, there isn't much difference between a custom content type and a content type: Both can be extended with fields.
There could be a difference between a content type created from a module (core module or third-party module) and a content type created through the user interface: the content type fields that aren't created from the user interface made available from the Field UI module. For example, the Forum module adds to the Forum topic content type a reference field for a vocabulary the module created. Similarly, the Standard profile adds a field to attach an image to nodes of the Article content type.