Hi, first time poster, and first time user of DEVONthink Pro (currently test-driving, though my hand is itching for my credit card 
)
As the title says, once I add materials into my database (doc, jpg, etc.), can I get rid of them permanently? Thanks in advance!
If you import documents into the database, you can delete them from disk. If you index documents, do not delete the original in the file system.
Of course, backups of everything are important - including the originals before you delete them, and the database(s) that you create.
Would dragging and dropping materials count as importing, or indexing?
Normal drag and drop (click-hold-drag-drop) is importing. You should see this icon 
beside the mouse arrow when importing.
Pressing option+command while dragging is indexing. This is the icon 
you should see when indexing.