When you add or import donations, those donations are -- by default -- added to the system as 'not yet thanked' donations. You can easily manage the process of thanking your donors in the system.
A standard dashboard item is called system alerts. It is found, by default in the Manage Data and the Finance dashboards. You can also add it to your main dashboard if you like.
Go to Finance or to Manage Data and you should see this dashboard object:
In the dashboard above, you can see there are 67 un-thanked donors.
To thank them, click the link and proceed through the 3-step process to send a donor a thank you