With a number of recent high-profile cases involving charges of bribery, it's a good time to answer this question.
As defined, bribery is the offering, giving, receiving, or soliciting of something of value with the intent of influencing the actions of a government official or employee.
Both parties can be prosecuted under these laws--even if nothing is ultimately exchanged.
A common misconception about bribery is that it involves only the exchange of money, when in actuality it involves the exchange of anything that the government official may find valuable.
This includes gifts, personal favors, discounts, jobs, and promises.
Because bribery is an intent crime, meaning that the person offering, giving, receiving, or soliciting a bribe must intend to influence or be influenced, it's not difficult to see what kind of statements and actions can lead to prosecution.
But what about campaign contributions and gifts? Don't people ply candidates and politicians with the intent of influencing official decisions?
Theoretically, yes. But campaign contributions are considered to be a form of free speech, expressive of one's political beliefs.
To contend with this dichotomy, in the Sun-Diamond Growers case, the Supreme Court found that giving a gift or contribution to a public official or candidate is not necessarily against the law. Instead, prosecutors must show an actual connection between the item of value and a specific past or future act.
In other words, prosecutors must prove that the donor actually wielded improper influence.
Now that you know the answer to "What is bribery?," hopefully you understand a bit more about why politicians can accept so much money. You may find it unfortunate, but it's the law.