Recently Twitter did a refresh to their Tweet and Follow Buttons and now after the release of the New Twitter they have introduced - #hashtag button and @mention button.
“The new #hashtag button tells your visitors there’s an interesting conversation happening on Twitter, and lets them join in with just one click. The @mention button encourages visitors to Tweet to your account, driving public conversation directly from your website.”
So to get started and configure your own button experiences check out the new Twitter Buttons website.
Select the Button you want , configure them, copy and paste the generated code
For example I have put a #Hashtag ButtonTweet #HashtagButton
And a @mention buttonTweet to @vasudevg
So its up to you, how and where to put to use these buttons. many new features are being added to the New twitter, So stay tuned !!