By default Magento includes the option to allow customers to tag your products. When a customer tags a certain product, the tag appears as pending and must be approved before it can show up on the product page.
Let’s add a tag to our product, approve it and see how it then shows up on the product page. To add a tag to a product, simply write the tag word in the Add Your Tags: field available on the product page and click Add Tags. We’ll add “Great” as a tag. A confirmation message will show up saying that the tag has been accepted for moderation
Now that the tag has been approved, all other customers will see it on the product page.
You can also manage tags on a per-product basis. Go to your Magento admin area -> Catalog -> Manage Products and click the product the tags of which you want to check. Then from the left menu click Product Tags and you will see all the tags for that product.