Toys R Us
Toys R Us (TRU) is an U.S. based big box retailer focusing on toys. At one point it had the largest share of toy sales in the U.S. but has recently been eclipsed by Wal-Mart. TRU has stores in many countries but probably has the most in the U.S. TRU is also one of the biggest toy retailers in Australia and the United Kingdom. For more information on Toys R Us, see the wikipedia article.
LEGO often provide "TRU Exclusives" or sets that are on offer only at TRU at least for some limited period of time. These are known to occur in Australia, the U.K. and the U.S.
Turnover of set ranges at TRU is slower than at Wal-Mart and other generalist toy retail outlets where you will likely find only the latest ranges. In the past, it was common to find sets 5 years old or older, hidden away in back corners of the shelving, because TRU did not discount. In recent years they have been more aggressively discounting to improve their turnover (although many feel they are not as good at it as Target and other general retailers).
TRU is not known for heavy discounting though, or for having much manager autonomy to haggle over damaged or large quantity purchases.
The term "TRU" seems to be something used more by ALEs than by the retailer itself or by other groups.
1 - The middle R in the name is, when represented graphically, shown reversed. It is sometimes but not always shown with single or double quotes around it when written in articles or headings. This seems to vary by geography, preference of the author and other factors that are unclear.