How were the voyages of exploration led by James Cook different from the earlier voyages of discovery undertaken by Columbus and other Spanish and Portuguese explorers?

James Cook's first voyage was for scientific exploration purposes. He was sent to the Pacific Ocean in support of the Royal Society's research into determining the accurate longitude of the Earth. Cook was a cartographer and captain in the Royal Navy.
In contrast, Christopher Columbus embarked on his first voyage to the Americas under the sponsorship and orders of the Crown of Castile. According to historical records, it appeared that Columbus was searching for "real estate" for the Catholic Monarchs of Spain. Simply put, Columbus was more of a colonist than an explorer. Christopher Columbus' landing in the Americas (present-day Bahamas) is what initiated the permanent European colonization of the Americas.
The other notable explorers from Spain and Portugal, such as Magellan, also had non-scientific objectives. For instance, many Spanish and Portuguese navigators were ordered to circumnavigate the world to establish trading routes and to make contact with cultures who possessed spices and minerals that could be sold for high prices in Europe. In contrast, a majority of Cook's voyages were scholarly in nature.