WorkSheet Selection Change and Vlookup

I have a sheet (Sheet1) that has a product code entered, Column C.
The product description is in the same sheet in Column D.

I set up a sheet to manually enter the product codes and their descriptions ( Product Codes).
Column A is the Product Codes, and Column B the Descriptions.

If I enter an Product code into Sheet1 Column C, and there is an already entered Product Code in the product code sheet along with a description, I want the description to display after exiting the sheet1 product code in Column D.
I cannot formulate the cells, as not 100% codes are manually entered, and the user needs to sometimes manually enter both the code and he description.
What code would be best to accomplish this?