URLs for links can be specified later in the document.
Markdown
[Text1][1] will link to the first link, and [Text2][2] to the second.
You [can reuse][1] names, and give longer names [like this one][a link].
You can also link text [like this] without giving the reference an explicit name.
[1]: http://www.google.com
[2]: http://stackoverflow.com/
[a link]: http://example.org/
[like this]: http://stackexchange.com/
Output
Text1 will link to the first link, and Text2 to the second. You can reuse names, and give longer names like this one. You can also link text like this without giving the reference an explicit name.