Loretta the Letter Lady is a series of short inserts that aired on Nick Jr. (back when it was a block on Nickelodeon instead of a channel in its own right) in the mid 1990's. Each episode took place in "Nick Jr's Alphabet Store" (which is situated at the bottom of a grassy hill with no other buildings around) and featured Loretta the Letter Lady showcasing a certain letter in her store (among many others on shelves behind her), explaining the sound it makes and giving examples of words that begin with the letter in question (with accompanying visual aids). On at least one occasion, Loretta also makes up a new word that starts with the letter she's explaining.
The only known footage of this show that exists online was published to YouTube on Jun 30, 2014; it is the episode where Loretta the Letter Lady showcases the letter 'T': http://www.youtube.com/watch?v=CA-WHn4i_L4