Credit rating firms such as Standard and Poor's, Moody's, and Fitch Ratings provide credit ratings for a company and its bonds. The term "investment grade" refers to the highest caliber bonds, which include those issued by the US government and extremely reliable businesses like several utilities.
"High yield" or "junk" bonds are those that are not investment grade but are also not in default. Investors demand a larger coupon payment on these bonds since they have a bigger default risk in the future.