Search engines may make the assumption that pages not linked to, or buried within a web site’s internal link structure, are less important, just as they assume that pages that are not linked well from external sources are less important than those that are.
Linking from the home page to content that you would like to rank can improve that page’s rankings, as well as linking to it from a sitemap and from various related content within the site.

In some cases, pages like “Terms & Condition, Help, FAQ, etc…” doesn’t go well with the main navigation links, so a good practise is to include them into the footer of you web site.

e.g. home | terms | help | faq...

Another example is when the links structure are created using flash or form select menu. 
Search engines are known for not being able to read images, flash content, etc. and they can not submit forms, so creating a sitemap and adding a link to it from the home page, or adding those links at the bottom of the page, helps search engines in finding to pages included within the web site.

Talking about forms submission.
    It’s a good practise to offer your visitors a search form, especially when you have an e-commerce website with 1000′s of products.
As I said above search engines can not submit forms, and so the search page is not that relevant to them, but there is a way in making it important and gain from it.
One solution I found to be very useful, is to log all searches done by users into a database or text file, and with a bit of coding you can create a large sitemap (over a period of time) that could turn your search page into a relevant source for search engines.

For instance Google Sitemaps. 
With Google Sitemaps using a XML format, you can inform Google of all pages you have on your website. 
XML Sitemap files can include settings for each URL as 
  :. their priority
  :. page last modified date 

Pagination, used for navigating through few pages related to a search or product category. Very useful, but can have side effects regarding search engines if not implemented right.
For example:

consider the following navigation: 

home page -> page1 -> page2 -> page3 ->page4 ->page5 ->...... 

or 

prev | next

The fifth page is harder to reach not only by users, clicking the least 5 times, but also by search engines, which can very easily treat the fifth page as less important then  the first one, also called “Death by Pagination“.
Ways around it:
  :. create a plain HTML sitemap with links to all the pages
  :. create an XML sitemap and submit it to the search engines for crawling
  :. create a better pagination like:

prev | 1 | 2 | 3 | 4 | 5 | next

It is likely that keywords contained by URL, both in the domain name or in the file name, do have a minor but apparently positive effect on ranking and plays a very important part into search engines optimisation.

It also likely has an effect on CTR (Click Through Ratio) because keywords in the URL may make a user more likely to click a link due to an increase in perceived relevance.

The URL, like the Page Title, is also often selected as the anchor text for a link.
This may have the same beneficial effect as mentioned in SEO Page Title

If your website is hosted on Linux, you can make use of the .htaccess file to create Rewrite Rule and redirect long query strings pages to nice looking, friendly URL’s.

Example:

Current URL could be

www.domain-name.ie/pagename.php?a=1&b=2&c=3

From a search engine point of view, this type of URL format tells nothing about the page itself, but this example should:

New URL:

www.domain-name.com/details-Product-Name-Here-a1-b2-c3.html

To achieve this type of url’s, create a new file and name it .htaccess and ad the following line at the very top of the page:

Options +FollowSymlinks

RewriteEngine on

RewriteBase /#new rule

RewriteRule ^details-(.*)-a([0-9]+)-b([0-9]+)-c([0-9]+)\.html?$ /pagename.php?a=$1&b=$2&c=$3 [L,NC]

#where L = last rule, NC = case insensitive, (.*) = anything, ([0-9]+) = numbers only

Now change the link format on your page as shown in the example above and you are done.

Search engines will evaluate the links that a document contains. A related link on a web page is valuable content in and of itself, and is treated as such be search engines.
However, links to totally irrelevant or spam content can potentially hurt the rankings of a page. 
Linking to a “bad neighbourhood” of spam sites or even lots of irrelevant sites can hurt a site’s rankings.

The point is if you have an exchange links page, always go back to it and verify that the sites your a linking to and from are still relevant to your content, and they are still live.

Is very clear that a page that contains the keywords a user is looking for should be relevant to their search query. Search Engines algorithms take this into account as well. Keywords included into the page content should not be done in excess, and mentioning the keywords in the form of singular, plural, past, present and so on is not beneficial and could also be treated as spam or what we like to call it  ”keyword stuffing“.

Not as important as it used to be. 
Many SEO experts says they are not used anymore by search engines to rank your site, and despite this some of them still use it.
As I was reading on Red Fly Marketing once, and I quote:

Well, over the years as meta tag keyword stuffing became prominent, search engines adjusted their algorithms to give less weight to the tags. A LOT less weight. Now, meta tag weight is so negligible that many webmasters no longer even use them.
You will notice that I use them here. The main reason is because Google will normally list a sites meta description ( Of course we all know this is query dependent ) as the description in SERPS. I would prefer to have control over this rather than let Google choose what to display.

he’s perfectly right.
For the most part, the importance of a meta description or keywords lies in the fact that search engines may choose to use it in the SERP’s, instead of displaying relevant bits from the page content.
A meta description might have a minor effect on search engines rankings, but is definitely not a critical factor.