Hi EGS,
I think there might be a little confusion with terms.
Simple MeaningAs stated above a 'landing page' just means the first page a visitor hits (lands on). With ppc you get to choose your landing page. So if you have traffic you technically have landing pages.
Common MeaningSince ppc is under your control, landing pages have become a big focus. Google and others are often encouraging you to have good optimised landing pages. So
landing page has become a sort of shorthand for: 'Landing page that has been created and/or optimised specifically to help convert paid advertising visitors'. That's usually what is meant by the above.
To confuse thingsTo confuse things, a lot of people also refer to SEO landing pages or something similar. That basically means pages specifically designed and optimised to draw organic visits (rank) & convert them. YOu also hear people saying things like 'every page needs to be landing page'. Which of course is true because visits can start anywhere, but confuses the term even more.
What people do with them As landing pages is just whatever page a user sees first there are no hard rules. But when people are giving them attention these are some characteristics of many landing pages:
1 - Clear heading/images/content & specific content - Tell a searcher they have reached where they need to reach (reduce bounce rate)
2 - Clear Calls to action, usually above the fold - Clear call(s) to action (doesn't have to be 'buy' it can be 'see more' or 'browse'.
3 - Reduced navigation - in attempt to reduce the # of choices and direct the visitor to where they want to go.
For eccomerce this might mean just putting together a category page that is super relevant to the search or it might mean a 'stand alone page' that I call introductory page. The idea is that someone searches for say 'checkered lederhosen' you give them a page with checkered lederhosen in or near the title, an image of checkered lederhosen & a few checkered lederhosen related links:
1- buy checkered lederhosen
2 - See photos of our staff in checkered lederhosen
3 - Join or newsletter to receive checkered lederhosen updates
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When should landing pages be used? Would it be appropriate to use landing pages for both visitor-to-customer conversion AND search engine optimization (through keyword density and other SEO tactics), or is it more appropriate for one use over the other?
You can, but not every landing page would be good for this. The reason for that is that optimising a page for SEO, also (not coincidentally) is very similar to trying to get your visitors (already on the site) to visit that page. So if the page doesn't make sense mid browse (like an introductory page) you may not want to do that.
What you may want to do is work on those pages that you see that SEO visits are landing on already (see 'top landing pages' in Google Analytics)
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Can and should landing pages be used for PPC-marketing and other marketing/advertising campaigns?
Yes, that is their main/conventional use.
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In an e-commerce world, the one thing I really like to see are visitor-to-customer conversion. A lot of traffic means nothing if you don't make the sales
Great. Landing page optimisation is mostly about making better use of the traffic you're already getting. Not so much about more traffic (as I said, that's sort of stretching the term)
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internet marketing vs. SEO, and which should be used, or whether or not both can be used at the same time, with sure effectiveness.
Well there is a good bit of synergy between them & they rarely (IMO) conflict.
Also, if one works for you the other probably will too. However, in some cases, usually due to your relative advantage in one or the other and/or competitiveness of the two, one will overshadow the other.
But I'd say that for e-tail, in most conventional cases, it is a good idea to have a bit of both.