This is called Scientific Notation, and Excel automatically converts numbers longer than 12 digits if a cell(s) is formatted as General, and 15 digits if a cell(s) is formatted as a Number. If you need to enter long numeric strings, but don't want them converted, format the cells in question as Text before you input or paste your values into Excel. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.