Paste this code into your page's HTML, where you want the button to go.
Why should I add Tumblr buttons to my site? Besides the aesthetic appeal, they're actually quite practical. We assume you want more site traffic. Tumblr posts created by these post buttons include permanent links back to your page, no matter how many times they get reblogged. Even if they get embedded on another site, they'll still have a link back to you. Your URL will be an unscrape-able barnacle on the keel of every post that sails from your site. Ahoy! Follow buttons let people know you have a blog. They already like your site. They'll probably like your blog—if you let them know about it.
What is the post button actually counting? Ah! It's counting all the posts it's created, plus all the likes, reblogs, and replies that those posts get. So if the button creates 10 posts and they all get 10 notes, it'll show 110.
Can I control which post type the post button creates? Indeed you can. By default, share buttons will create a photo post (or a link post, if there aren't any suitable images) but with a bit of configuration you can make the button do exactly what you want. If you plan to configure the button for audio and video content, you should know that the player has to be whitelisted by Tumblr. If you're not using YouTube, Vevo, Vimeo, Spotify, Soundcloud, or one of the other big guys, your audio or video post might be converted into a link post when it hits the dashboard. You'll find all the documentation you need here.
Buttons are great and all but I really need an API to do what I want to do. Know what I mean? Sure, pal. We know what you mean. You've got a mobile app, or you want to do something wild with your website. Here's all the documentation for our API. It should cover anything you have in mind.
If someone makes a post using a post button on my site, will they start following my blog too? Sorry, no. Each button does one thing and one thing only. If you want people to follow your blog, you'll need to put a follow button on your page too.