A Delaware corporation is a corporation chartered in the state of Delaware in the United States.
Delaware is well known as a corporate haven, and many major corporations are chartered in Delaware because the state charges no corporate taxes not operating within the state and has laws and courts that are seen as friendly to corporations. Because of the large number of major corporations chartered in Delaware, the courts in that state are generally regarded as more experienced in the application of corporate law than the courts of other states. Disputes over the internal affairs of Delaware corporations are frequently filed in the Delaware Court of Chancery, which is the only separate court of equity (as opposed to law) in any U.S. state. Because it is a court of equity, there are no juries, and its cases are decided by the "judges" whose actual title is "chancellor" (the chief one) or vice-chancellor (the others, of whom there are currently four).
Despite having their corporate headquarters in the Silicon Valley, many Californian companies incorporated in the state of Delaware seeking better protections against hostile take over. For example, SGI was incorporated in California in 1981, but it reincorporated in Delaware in 1990. This is mainly due to differences in corporation laws among the states.
A similar strategy is used for financial institutions. Many U.S. States have "usury" laws limiting the amount of interest a corporation can charge, but Federal law allows corporations to 'import' these laws from their home state. Delaware (amongst others) has relatively lax interest laws (that is, allows banks to charge as much as they want), hence the preponderance of credit card companies in the state.