Much of the confusion about Blu-ray has been due to Blu-ray player "profiles." Every BD player falls into one of three profile categories, which roughly describes how the player handles (or doesn't handle) special features.
When Blu-ray was first introduced, the Blu-ray Association did not initially require all players to support all of the format's advanced functionality. In consequence, Profile 1.0 and Profile 1.1 Blu-ray players can't play back some of the bonus content found on newer Blu-ray Discs. This may not be a deal breaker for everyone, since many people just don't care about special features. We'll also note that we've been underwhelmed with almost all of these kinds of features so far. That said, profiles are an unavoidable and potentially confusing aspect of Blu-ray, so here's what you need to know.
Profile 1.0: This guarantees the player can play back Blu-ray movies. Profile 1.0 players can also access standard audio commentary tracks and the typical kinds of special features you'd find on a regular DVD. Even if you don't care much about the new Blu-ray features described below, we'd recommend against buying a Profile 1.0 player. They tend to be much slower and buggier than newer Profile 1.1 and 2.0 players.
Profile 1.1: Profile 1.1 adds picture-in-picture functionality, also known as BonusView. This is usually a video of a director giving commentary or other behind-the-scenes footage that plays in a small corner while the main movie plays in the background. As of late 2008, numerous Profile 1.1 players are still available from many major retailers and are worth considering if you don't care about Profile 2.0 features and you're getting a good discount.
Profile 2.0: The latest, and supposedly last, Blu-ray profile enables playback of "BD-Live" features. BD-Live refers to any content that needs to be downloaded from the Internet, which means you'll need to connect your Blu-ray player to your home network (typically via an Ethernet cable) to enjoy these features. BD-Live features generally consist of additional deleted scenes or outtakes, games and quizzes, trailers, pop-up text commentaries, and message boards. To be blunt, most existing BD-Live features are pretty unexciting and are more of a gimmick than anything else. Still, it's possible that these features will improve as content makers get a better handle on the technology. One of the more innovative BD-Live-enabled discs, The Dark Knight, allows users to generate and upload their own picture-in-picture commentary tracks.
The safest bet: If you're paranoid that there are going to be more Blu-ray profiles or other changes in the future--and we wouldn't blame you--the safest bet is to go with the PlayStation 3, which is the most upgradeable Blu-ray player and typically the first to get new features. That being said, we don't foresee any additional Blu-ray profiles, so you're probably future-proof if you go with a Profile 2.0 player.
If you're looking for more nerdy details about the difference between Blu-ray profiles, check out our full Blu-ray profile guide.