The short-sleeved polo is a gentleman's t-shirt; it's no more expensive and just as comfortable than the ubiquitous tee, but it looks so much better. It should be a staple of your spring and summer wardrobe.
In this case, polo refers not to the Ralph Lauren brand, but to a style of shirt that was actually invented for tennis by player Rene Lacoste around 1926. Polo players adopted the style in the early 1930s; for whatever reason, it quickly became known -- even among tennis players -- as a polo shirt. The style was adopted by golfers over the next generation too. So, while it's most popularly called a polo, it's also often referred to as a golf or tennis shirt. Ralph Lauren named his new company "Polo" when he started it around 1967, and introduced his popular and excellent (and expensive) version of the polo shirt about five years later. I guess his is a Polo polo.
I've sworn by Ralph's version for years, always buying them at places like Name Brand Clothing, Stein Mart, T.J. Maxx or Polo outlets for $20-30, as opposed to the $65+ they go for at retail. The main difference I found between Polo polos and cheaper ones is the collar and placket, which looks neater because it stands up on its own, even unbuttoned, while cheaper ones fall open and look sloppier.
But the past few years it's been harder and harder to find them at discounters, so I started looking for other options.
Target Merona Ultimate Polo
So I tried Target's Merona polo for the first time last year. I wound up buying about 10 of them in different colors. They're logoless, which is a plus to many purists, and 100% cotton. They retail for $13, and frequently go on sale for $10. They're excellent for that price, and I plan to buy a few more this year.
J.C. Penney St. John's Bay Polo
I wanted a burgundy and a dark green polo, and Target didn't make them. But J.C. Penney does under their St. John's Bay brand, so I tried them too. They're also logoless, 100% cotton, and $13, frequently on sale for $10. They're also quite good for the money, although a little snugger in the same size as Target's. In my opinion, Target's is better for more casual wear as it's a little looser, while Penney's slightly snugger fit is better to tuck in and wear with an odd jacket or blazer.
(Incidentally, Target has introduced an olive green for this year; the one I bought from Penney's was more of a bottle green.)
If you're looking for new polo shirts for the coming warm weather, try J.C. Penney and especially Target. They're on par with Polo polos, especially given that they cost less than 1/3 as much even when Polo is deeply discounted, and I haven't noticed the sloppy placket problem with either. You can put the money you save toward something else in your closet, something where the difference in price means more than a marginal difference in quality.