A) to add tags directly when entering/editing a product, you will need an extension, such as this one. This method is most useful to let people with less magento experience create tags (such as data entry people).
B) To add tags without leaving the administrator (not going to the frontend). This is useful for applying the same tag to multiple products:
Create tags through Catalog->All Tags-> Add New Tag button.
After creating the tags, click on each tag. You’ll see a detailed set of options to “Edit Tag”.
Go to the “Products tagged as administrator” tab, click on ‘Reset Filter” . You see all your products getting listed.
Select the products to which you want to assign the tag & click on “Save Tag”.
C) To add tags via the frontend & the backend, which is useful when you want to add a lot of tags to each single product:
Just create your products as normal. When you are ready to add tags to your products just create yourself a customer account for your store and go to each product and add them as a customer would. I wrote out all of my tags in a text editor. Now since I am using phrases, and not just words, my tags will be entered as:
‘Stylus C64’ ‘Stylus C66’ ‘Stylus C84’ ‘Stylus C84N’ ‘Stylus C84WN’ ‘Stylus C86’ ‘Stylus CX3600’ ‘Stylus CX3650’ ‘Stylus CX4600’ ‘Stylus CX6400’ ‘Stylus CX6600’
Take note that you need the single quote (’) to bracket your phrases… or just spaces between each word if your tags are single words. Very easy to write out and then copy/paste into the Add Your Tags section of each product.
Once you copy/paste your tags in the front-end you will be prompted with a message that they must be approved from the admin/moderator. Now log into the back-end and go to the tags section (Catalog > Tags > Pending Tags). There you will see the list of tags you just submitted from the front-end of the site. Now just click “Select All”, then from the Actions drop-down menu choose “Change Status”, and then from the status drop-down choose “Approved” and then “Submit”.
Now you can pick a method most appropriate for each type of use!
http://stackoverflow.com/questions/9261935/add-tags-when-creating-product-in-magento