This is what Twitter said today:
"Starting today, we're rolling out a new font on twitter.com , moving from Helvetica Neue to Gotham."
They further explained:
"Words don’t just hold meaning; they communicate by their very form. We primarily use the Gotham font family: elegant and direct, stylish but not exclusive. Putting well-designed words in our product enhances the user experience,"
Helvetica Neue is from the Helvetica font family, it is sans-serif typeface that was developed in 1957 by Swiss typeface designer Max Miedinger along with Eduard Hoffmann. It was originally known as Neue Haas Grotesk, and in 1960, it's name was changed to Helvetica (meaning Swiss in Latin).
Gotham is a also a sans-serif typefaces that was designed by Tobias Frere-Jones (American type designer) in 2000. It was inspired by a form of architectural signage that was famous in the mid-twentieth century, and were especially popular in New York City. Infact the word Gotham is a nick name for the New York City, along with the Big Apple, The City That Never Sleeps and Empire City.