Deactivation occurs at a user's own discretion, is reversible, and their hovering account details (what you see when you hover over a username) are viewable.
Deletion must be performed by a staff member at the request of a user (usually. I imagine they can also delete an account forcefully if the user earns a permanent ban), is not reversible, and their account is completely inaccessible (nothing will appear if you hover over their username).
You typically cannot view a user's profile in either case, but deactivated users will still have clickable usernames.
Example of a deactivated account:
@xiao
Example of a deleted account: @karenina