The +1 button lets people share your content to Google+ from search results or from your site itself. (Learn more about adding the +1 button to your site.)
A +snippet is the text that Google displays when somebody shares your content on Google+. For example, here’s the +1 snippet that appears when Timothy shares the page www.youtube.com from the +1 button on the page:
You can customize exactly what appears in the +snippet by specifying the name, image, and description to be displayed. For example, if your site lists movie reviews, you can ensure that your +snippet for each movie page includes the movie's title, poster, and short description. If you have a shopping site, you can make sure that your +snippet includes the product name, image, and price.
To customize your +snippet, mark up your source HTML using the schema.org 
vocabulary. This helps Google understand your content and create the most useful +snippet. You don’t need any special knowledge to do this, just a basic understanding of HTML.
results. (For more information, check out these articles about rich snippets and schema.org.)Each +snippet can contain a name, an image, and a description, so make sure to include each of these properties in your code. (If you don't specify an image in your markup, no image will appear in your +snippet. Also, the +snippet may not include specified image if that image has an aspect ratio or size that does not display properly in the stream.) Here’s an example of how you could use schema.org to mark up an article on your blog:
<body itemscope itemtype="http://schema.org/Article"> <h1 itemprop="name">This is the article name</h1> <img itemprop="image" src="thumbnail.jpg" /> <p itemprop="description">This is the description of the article.</p> </body>
Google+ will use the name, image, and description properties to create a +snippet.
If you haven’t marked up your content using schema.org, Google+ (though not Google Web Search or Google Custom Search) will look for Open Graph markup or title and meta description tags. If none of this information is present, Google will parse the page and attempt to find the best title, description, and image.
More information about customizing the site description.