Links are the web addresses. The page that you are reading right now has a web link, the link that you are seeing on the browser address bar. Every link is unique. What that means is that a link cannot point to more than one web page at a given point in time. Along time scale it might be pointing to different pages, but at a point in time it always points to one web page. A web page however may have multiple web addresses.
A link is specified by HTML anchor tag. For example, if you want to link back to my home page, than the tag would go as follows:
<a href="http://www.lalitbhatt.net">Lalit Home Page</a>
We can speficy many other attributes to the anchor tag. A complete list can be seen at http://www.w3schools.com/tags/tag_a.asp
The tags that are imporatant from seo perspective are:
hreflang = Language of the linked web page. This is a two letter language code.
<a href="url of a web page" hreflang="fr">Page</a>
The hreflang="fr" tells that the linked page is in French. If you have a translated page for current page, than you need to use the link tag. For example, if translated page for current page exists in French then it is mentioned as:
<link rel="alternate" hreflang="fr" href="http://fr.lalitbhatt.net/" />
rel - This has many values that can be specified. The important from seo perspetive is 'nofollow'. If Google spider while indexing a page, encounters a link and sees nofollow attribute, it does not considers the current page providing an outbound link. So if you are looking for getting inbound links to your page, you would like them to not to have rel="nofollow".
From Google perspective, the other things that you might want to take care of is:
- If you are putting a paid link or advertisement not using one of the ad networks like adsense, the advertisement or paid links needs to be put with rel="nofollow" attribute.
- If you are paying money to put your link on other websites, you need to get rel="nofollow" attribute there also.
The basic tenet is that any activity which is an attempt to affect your search ranking, Google search spider hates that and will result in bad rankings or at worse complete ban.
Now let's look into some of the linking terms that we hear often
Inbound or Backlinks or Incoming Links
This is one of the most important term in Google world. It represents that how many web pages are carrying the links to your web page. Google considers it's one of most important factor to rank a page higher. The basic idea is that if so many web pages are putting a reference to your web page, the page must be damn good. After reading this please do not go and start buying links. There are infinite companies in this business selling you all sorts of page ranks and links from highly ranked pages. Buying links is considered as Link spamming and a ban might happen.
You can see the list of inbound links to your website in Google webmaster portal.
The other term is outbound links. These are the links that you put as a reference in your web page. For example the link to W3Schools web page that I have put at the initial part of this post. The page that you link to gets the point for being linked. Your outbound is inbound for the linked page.
This happens when your web page puts a link of another web page and the other web page puts the link for yours. Two to tango. This was a hot seo technique in olden days but not any more. Google does not gives any credit for this to either parties. It might still be important from business perspective, so use it if you need to do it and if that helps in driving customers.
The other concept that you want to be comfortable with is canonical url. It may happen that the same web page in your web site could be reached through multiple web addresses sometimes because of dynamic parameters. one simple example is links with and without www. For example http://www.lalitbhatt.net and http://lalitbhatt.net will point to same web page. However for search engines these are two different web addresses. Sometimes dynamic parameters are added to avoid caching issues however search engines consider them as separate addresses.
It's important is those cases to make sure that you tell search engine which is the URL that you want to consider as the primary url also known as canonical url. This can be done by using the link tag and make sure to put in head section of all the variation of the page.
<link rel="canonical" href="http://www.lalitbhatt.net" />
You can follow Google Webmaster Guidelines.