Here is the definition of Doorway pages by Matt Cutts

Doorway pages are typically large sets of poor-quality pages where each page is optimized for a specific keyword or phrase. In many cases, doorway pages are written to rank for a particular phrase and then funnel users to a single destination.

Now I am trying to define it in simple words:

Doorway pages are low -quality web pages with very little content but are stuffed with keywords and keyphrases. They are basically created to rank in searches. These pages are also known as entry pages, gateway pages, bridge pages, portal pages, zebra pages, jump pages. They don’t serve the purpose of visitor looking for information. They send visitors to a different page may be home page. Over the last year or two, search engines have changed their attitude towards doorway pages and are weeding out doorway pages from their indexes.

Reason for weeding out:

Many Webmaster create doorway pages and fill them with cloud of keywords. They do so to redirect visitors from them to the main site. Now, search engines are aware of this strategy or you can a problem, and they are penalizing such sites that are using automatically generated doorway pages. Some dishonest webmaster create a doorway page that is a copy of high ranking page to cause the search engine to detect the page as a duplicate and exclude it from the search engine listings.

Landing page and SEO

Landing pages are the pages that appear when a visitor clicks on an advertisement. These are common in internet marketing campaigns, especially email and PPC marketing management. The page will usually display content that is relevant to the feature specific keywords or phrases and that is optimized to those keywords or phrases for indexing by search engines.

In pay per click (PPC) campaigns, the landing page will also be customized to measure the effectiveness of different advertisements. Main aim of a Landing Page is to optimize the user experience when visiting a site.

In other words you can say that Landing Page is the portalisation of a keyword category that provides an immediate call to action for the general keywords being targeted - but provides additional routes to related niche pages with their own call to action.

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But every campaign needs to be managed and this takes a lot of time. Big wars get very heated, big  gaps can appear many times in just one day. Do you want to risk overpayment or loss of listings?  Do you  to get frustrated over fragmented billing? Or do you want experts to handle that for you too? Do you have time to handle thousands of keywords across multiple search providers?

1. Using geo-modified keywords in geo-targeted campaigns because this can result in a lot less traffic, since there is a strong chance the ads will not display due to a “geo-targeting mismatch.” One solution for this: use geo-modified keywords only in campaigns not using geo-targeting.

2. Duplicating keywords across ad groups or campaigns .When you have a good keyword, it may seem like a smart move to spread the goodness around to all of your ad groups and campaigns

3. Jumping the gun “All we are saying is give bids a chance.” Many times, we’ve seen advertisers make bid adjustments, or pause or delete keywords well before enough data has been collected to make an educated decision.

4. Don’t Putting your phone number in your ad . However, a phone number in your ad can decrease your click-through rate and quality index score, which in turn drives up your cost-per-click and pushes the ad further down in results.

5. Bidding on competitor’s keywords.Nope, it’s not OK to see what your competitor is using and then simply copy them. Doing this will result in your irrelevant keywords being removed by our guidelines review team, and could have legal consequences, as well.