Getty Images Commuters on a train to London looking at their mobile phones and iPad texting and playing games

Over the last couple of years, through the revelations of Ed Snowden and independent reporting by others, we've learned more and more about the National Security Agency's spying programs. Indeed, there have now been so many revelations that it can be hard to keep them straight. So here's a handy guide to the most significant ways the NSA spies on people in the United States and around the world.

This was one of the first programs revealed by Snowden and it continues to be one of the most controversial. The Patriot Act allows the NSA to obtain business records that are relevant to terrorist investigations. The government claims that this gives it the power to obtain records phone number dialed, time and duration of call aboutevery domestic phone call in the United States. Last year the Obama administrationproposed changes to require judicial oversight of access to the database.

A slide disclosed by Snowden lists 9 major internet companies Microsoft, Yahoo, Google, Facebook, PalTalk, YouTube, Skype, AOL, and Apple as participating in thePRISM program. The program allows the NSA to get private information such as emails, Facebook messages, and stored documents. It's not known how carefully these information requests are scrutinized.

Tailored Access Operations is the NSA's elite hacking unit. While some other NSA programs collect information in bulk, TAO engages in targeted attacks on high-value targets. It is believed that the NSA has a large library of exploits, allowing it to hack into a wide variety of consumer gadgets and business IT systems.

The NSAworks with countries around the world to tap into underseas fiber optic cables carrying vast quantities of fiber optic data. There's also evidence that the NSA has beentapping into fiber optic cables in the United States.

When you log into GMail, you'll see a "lock" icon indicating that communications between your computer and Google's server is protected by encryption. But until recently, Google didn't employ encryption when it moved data between its own servers. The NSA tapped into these connections and harvested large quantities of user data. Yahoo was also targeted.

Documents released by Snowden suggest that at least 35 world leadershave been targeted by the NSA, includingBrazilian president Dilma Rousseff and Mexican president Enrique Pea Nieto. There were also allegations last year that the NSA spied on Germanchancellor Angela Merkel, though a subsequent investigation cast doubt on that claim.

The NSA tapped into communications systems in Brazil andGermany and likely other countries as well to collect information about ordinary peoples' phone calls and emails.

The NSA has spied on cell phone networks around the world, collecting5 billion records per day about the locations of users' cell phones. The agency isn't allowed to deliberately target cell phone users in the United States, but some American cell phone records have been collected "incidentally."

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13 ways the NSA has spied on us