Opera is a popular browser with some of the best support for standards.
The faster web browsing is thanks to a new rendering engine, this is the software used to create the web pages. It also claims to support web standards properly including the Acid3 test (click the link to see how your current browser fares).
Acid3 is an extreme test, a more visible effect will be the support for web fonts. Most browsers can only use the fonts installed on the computer. Web designers often give the browser a list of fonts. The font they would actually liked used, a similar but more common font and then let the browser choose its own serif or sans-serif font. Web fonts let the browser load the font from a website.
Auto-update is one improvement we are pleased to see as it is a simpler process than having to download a installer.
New in Opera v25: Visual bookmarks - Integrated in-browser PDF viewer - Notifications in-browser to things like new email - Better access to web content like MP3s
Note that this is Mac OS X version of the browser.