What attracts you to click an ad on a website?

One single attractive message is enough to make your audience click on the ad.
Have you set that single objective you want to achieve through your ad? Have you designed your ad having this objective in mind? Have you selected the appropriate media for your ad? Have you targeted your ad according to the primary audience you want to reach? Have you planned for different executions so your ad does not become a wallpaper? Is your ad rich in its simplicity? if you can answer "yes" to all of the above then you can classify your ad as "attractive".

The design as well as the approach.
it should be interesting, unique and most importantly easy to understand which means using simple words and ideas.

Design Simplicity in design attracts me, there is a big chance the site is busy with content, so simple design for ads makes great contrast to catch my eyes, also I like the ads that expand the experience before clicking, like when the ad frame gets larger when you hover over it, it can be more interactive, and it can deliver more information.
Language Direct and smart language attracts me, I like simple (not too many) words that deliver the message of the ad and tells me a little about what will happen after I click on the ad.
Recognition If I can clearly see a logo in the ad, and if I recognize the brand, this will make me not scared of the ad, If the brand is new for me and I can't recognize it I will find it hard to click, unless the brand is defining itself well in the ad.
Relevancy This is maybe the most important point, definitely there is more chance I will click on the ad if it's relevant to things I am specialized at, things I want to buy, news I like to follow, etc.

I guess this is the $1 million question of advertisers! For me, it's pretty straightforward -- I will click if I think that I may be interested in the product or service behind the ad.
I certainly find Google search ads the most likely to be relevant -- especially when my search happens to be for a product; e.g., a hotel or flight.
I find myself not clicking on sites like facebook or other websites where I am not in a state of looking for something.

I personally think it's a combination of appealing colors and images and a very catchy line.
All this should be displayed in a very clear and eye-friendly way.

Hi Suraj Gill, Interesting Topic! As Omar said - I will Click the Ad if feel Interested in the Product / Service behind the Ad.
But however, before Clicking the Ad I would certainly consider Peggy, Ghady, Cynthia and Sa'ed’s Comments.
- Attractive Message - Should be Interesting, Unique and Easy to Understand with Simple Words and Ideas.
- Should be blend of Pleasing Color scheme, Images and Appealing Punch-Line.
- Branding Perception

Actual facts in the banner will make me want to click, such as prices and specs of a product, not some daft tagline.

The key for any successful online campaign is targeting, i wouldn't click on an Ad unless it is relevant to my interests.
Then comes the massage, and i prefer it to be whether simple and to the point or something that arouses my curiosity and makes me want to know what is behind this Ad, and of course the design is important to grab my attention to check the Ad in the first place

Graphics will capture my attention and obviously the tag line has to be attractive if I want to "Click" ...
so it is all about being "Creative"