This is commonly known as artcard.  300gsm is the thickest artcard that can be used with Laser Printers.  Like the 250 gsm counterpart, this paper is tough and is commonly used to make covers of booklets, posters, and namecards. 
 
Although much thicker, this card is also coated and will project printed colours well to give your documents and artwork that professional look using your very own laser printer.
 
Being in the design and print production industry for a long time, we recommend this paper for those who own a laser printer and seek to print their own booklet covers, posters or even namecards.  It is also printable on both sides and will work equally well with monochrome or colour laser printers.  (Warning: please check on the specifications of your laser printer to see if your printer can accept 300 gsm papers before proceeding.)
 
This paper is fantastic for students, entrepreneurs, and executives who need great laser-quality print output in manageable quantities.
 
Example of use:
A4 size two-sided documents, flyers or posters
A5 booklet cover
Namecards