If you want to download the photos or videos later there are two ways to do this:
Open the album containing the images you want to download and click on the desired image. Click on the three dots (…) icon right top. Choose 'download original'.
To download a video click on the photo before or after the video and navigate to the video with the arrows to open it in full screen. Now you can click on the three dots to download the video.
The whole album:
You can download albums by clicking on the three dots top right in the album you wish to download. A menu will open, choose 'download'.
Downloading by others
You decide whether or not content can be downloaded from your albums (photos/videos). Manage this on a per album basis in the 'album settings' or change the settings for all your albums in your profile.
The album owner can always download his own pictures (also when downloading is disabled).