Exporting allows you to save part of your collection as a text file or packaged Anki deck. To export, click the File menu and choose 'Export'.
If you choose "Notes in Plain Text", Anki will write the contents of the notes into a text file. Each field is separated by a tab. If you edit the resulting file and don't modify the first field, you can later import that file back into Anki and Anki will update your notes based on your edits, provided you import back into the same note type.
If you find yourself needing to edit the first field as well, you'll need to change the format of your note type so that the first field is an ID number rather than actual text. (You can install the Add note id add-on to make this easier.)
In order for formatting to be preserved when you import text back in, the text is exported with all the HTML formatting embedded in it.
A 'packaged deck' consists of cards, notes, note types, and any sounds or images bundled up into a file ending with .apkg or .colpkg. You can use packaged decks to transfer cards between people, or for backing up parts of your collection.
There are two different kinds of packaged decks.
When you export all decks with scheduling included, this is called a 'collection package'. Anki will copy your entire collection into a file ending in .colpkg, and place it on your desktop. A collection package is used to back up your collection, or copy it to another device.
Collection packages created with previous versions of Anki were called collection.apkg.
When this file is later imported, Anki will delete all the current cards in the collection, and replace the collection with the items in the file. This is useful for copying your collection back and forth between devices.
Existing media in your collection is not deleted when you import a collection package. To delete unused media, use Tools>Check Media.
If you choose Anki 2.1.50+ Collection Package format, imports and exports will be faster, and media files will be compressed, but the resulting .colpkg file will not be readable by older Anki clients.
Deck packages contain a single deck (and any child decks it may have). They have a filename ending with .apkg, but a filename other than collection.apkg. When you import a deck package, Anki will add the contents into your collection, rather than overwriting your collection.
If some notes in the deck package have previously been imported, Anki will keep the version with the most recent modification time. So if you download an updated deck, the edits that have been made in the updated version will be made in your collection as well, but if you re-import an unchanged deck after making edits in your collection, the changes in your collection will be kept.
If you choose not to include scheduling information, Anki will assume that you are sharing the deck with other people, and will remove marked and leech tags so that they will have a clean copy of it.