A henley a shirt is characterized by its collarless appearance along with a small row of buttons used to open or close the neckline. The closest approximation of a henley shirt would be a traditional polo shirt without a collar.
Henley shirts derived their name from their origin as a uniform top for rowing crews in Henley on Thames, England in the 1990s. Since then they have gained worldwide popularity both as a uniform peace among athletes and individuals looking for a comfortable casual piece of clothing that is more dressy than a T-shirt yet less dressy than a polo shirt.
Henley shirts are also popular due to their perceived roominess as they are oftentimes constructed to be less formfitting then a T-shirt. While this is not true of all henley shirts as some are cut to be skintight; it is more common to find loose varieties.