The Gallery is a "private album" that is accessible only to the client and you via a link; it is not displayed on the website.
The Portfolio is a set of albums that you can choose to publish or not publish on the website. This means that by adding an album in the portfolio section, you can publish and show it on the site for clients who are not yet familiar with your work.
The distinction between the gallery and the portfolio is that:
in the gallery, both you and the client can download files
in the gallery, you have the opportunity to see small statistics directly on the page, while information about portfolio views can be seen on the main page (under "Top Pages" analytics)
The similarity between the gallery and the portfolio is that:
in both cases, you have the option to add video (on a paid plan) and photos
you can customize the design of the album cover
you can upload photos and videos in the highest quality