A tag is term assigned to a piece of information or a non-hierarchical keyword. They,re chosen by the items creator (of the bookmark, image, computer file or article) as a way of describing, organising or categorising the item. It makes it easier for others, but in some cases the author to find the content quickly and easily. It has become an important feature of Web 2.0 services (Web 2.0 is just tech-speak for websites where anyone and everyone can participate). The term "Web 2.0" was first coined by Darcy DiNucci in an article titled "Fragmented Future" published in 1999.

• Tags help with the internal HubPages search.
• Tags help determine what text ads appear on your hubs. The more specific, the more relevant the ads should be.
• Every tags page links back to your hub. Popular top-level tags like technology, entertainment or finance are seen on HubPages's front page.
• Tags will help position you in other related hubs (of other Hub Authors) under the Related Hubs on the right hand side of the hub.

• Tags are keywords. What keywords describe your content?

• Only use tags that are appropriate to the context of your content.
• Use the Google Keyword Tool to find suitable synonyms, phrases and keywords related to the term that you're using. But don't go crazy. Only use the ones that are appropriate.
• Smelling pistakes are acceptable! People make typos when doing searches.
• You need at least four, or otherwise you'll trip the 'not enough tags' filter.
• I do less than ten. Later I'll sometimes add more. 10-20 is okay. More than 20 is probably overkill. And search engines might see it as keyword-stuffing. Though a 1000+ word article would justify up to 30 tags.
  

Blogs have always allowed for categories, and a blog post being able to be listed across multiple categories. Bloggers can often confuse the two.

Categories should be kept to a workable minimum. Depending on the manner in which catregories are displayed in the blog softwares sidebar or, particularly, the top menu. It can get somewhat crowded.

Generalizing categories will work to your benefit. For example, if I were to make a blog about weight training and muscle groups I could have biceps, triceps, forearms... I could generalize that as arms.

Calves, quadriceps and hamstrings could be generalized as legs. I might even put the gluteus maximus in there. As well as any references to thighs.

Abdominals, obliques and intercostals could be generalized as torso or stomach.

Lats and lower back would be back.

Deltoids and trapezius would be shoulders.

Chest... that's pretty much all by itself. But I'd probably throw serratus in there too.

That's six categories. That'll fit neatly in a top menu. Rather than 15 or 16.

Tags on the otherhand, you could go crazy with the tags.

If blogging about the excerise 'torso twists' which works the obliques, you'd use the cateogry 'stomach' but could use tags such as torso, torso twists, external obliques, internal obliques. I'm sure there are more.

The blogging software will automatically generate tag pages. Each of your tags will be a hyperlink an index page listing all of the blog posts associated with that exact tag. This helps organize and allows you to cross link related posts. Users are two clicks away from finding more information of the same theme.

Tweets on Twitter can be tagged by including one or more hashtags. These are words or phrases prefixed with a hash symbol.

Mashable put it best when they said "...hashtags help spread information on Twitter while also helping to organize it."