When to use # to get object in javascript

[apex944]

I know people will down vote this, but I always used it but I don't know why.Sometimes I need to get objects like this:\[code\] var object = document.getElementById('ObjectName');\[/code\]And sometimes like this: \[code\] var object = document.getElementById('#ObjectName');\[/code\]What's the difference between them?I use Asp.NET