From the Organizer or Settings page, you can export Spreadsheets of your inventory as CSV files, which can be opened in spreadsheet software like Excel or Google Sheets. The format of this spreadsheet will change as DIM changes - if you're building tools around this spreadsheet, consider going directly to the Bungie.net API instead, or be prepared to react to changes in the format.
Importing Tags and Notes
If you have a .csv file containing columns labeled
Notes (and any number of other columns that are ignored), you can re-import the file "Import tags/notes from CSV" button on the Settings page. This will overwrite the tags and notes for each item in your spreadsheet.