Over a hundred years before a monstrous array of vacuum tubes surged into history in an overheated room in Pennsylvania, a properly attired Victorian Gentleman demonstrated an elegant little mechanism of wood and brass in a London drawing room. One of the ladies attending this demonstration brought along the daughter of a friend. She was a teenager with long dark hair, a talent for mathematics, and a weakness for wagering on horse races. When she took a close look at the device and realized what this older gentleman was trying to do, she surprised them all by joining him in an enterprise that might have altered history, had they succeeded.  Charles Babbage and his accomplice, Lady Lovelace, [Ada Byron, daughter of the Romantic poet, Lord Byron], came very close to inventing the computer more than a century before American engineers produced ENIAC (see note)** 

Even though  the "Analytical Engine"   [Babbage's machine] was yet to be  built,  Ada experimented with writing sequences of instructions [for it]. She noted the value of several particular tricks in this new art, tricks that are still essential to modern computer languages--subroutines, loops and jumps...  Ada created the loop--perhaps the most fundamental procedure in every contemporary programming language.  It was the conditional jump that brought Ada's gifts as a logician into play. She came up with yet another instruction for manipulating the card-reader, but instead of backing up and repeating a sequence of cards, this instruction enabled the card-reader to jump to another card in any part of the sequence, if a specific condition was satisfied. The addition of that little "if" to the formerly purely arithmetic list of operations meant that the program could do more than calculate. In a primitive but potentially meaningful way, the Engine could now make decisions. 

While Ada Lovelace has been unofficially known to the inner circles of programmers since the 1950s, when card-punched batch-processing was not altogether different from Ada's kind of programming, she has been relatively unknown outside those circles until recently. In the 1970s, the U.S. Department of defense officially named its "superlanguage" after her. 
-- from Tools For Thought: The People and Ideas of the Next Computer Revolution  by Howard Rheingold

The Bride of Science: Romance, Reason, and Byron's Daughter by Benjamin Woolley is an interesting and entertaining biography of  Ada Byron, the Countess of Lovelace.

** In 1835,  Charles Babbage unveiled  plans for  his  Analytical Engine.   Babbage's machine had many of the components of a modern computer: a control unit (punched card control), memory (the store), a processor (the mill), an input device (cardreader), and an output device (a printer). Although the Analytical Engine was never built, Charles Babbage's design was a  precursor for the modern computer.