Altcoins are digital currencies introduced after Bitcoin. In general, they present themselves as better alternatives to bitcoin. The success of Bitcoin as the first digital currency led many developers to introduce new digital currencies with general or minor changes.
Many altcoins are built on the basic bitcoin framework. Thus, most Altcoins are similar in this respect, most Altcoins are offered with a change in the extraction process or the way transactions are approved. But even with many similar features, Altcoins are widely different.