tier 1 are links that point directly to your site
tier 2 are links that point to tier 1 links
tier 3 are links that point to tier 2 links
etc...
And strategy how to build them?
well, you'll obviously want the best and most powerful links you can get to point directly to your site so you can reap their entire power (authority) directly.
The best and most powerful links (so, tier 1) are usually:
- quality PBNs
- quality guest posts
- quality niche edits
- quality PDF submissions
- quality forum links
- quality edu / gov links
- quality social media links
The tier 2 links can be anything you want (including the aforementioned quality links if you wish to save these / some of them for tier 2), and are usually used to power up the tier 1 links by infusing extra authority into them. Or, some people use tier 2 links to simply index tier 1 links when the tier 1 links - despite of their quality - for some reason won't index.
Tier 2 can be anything you want, like I said, but they're usually these:
- cheap PBNs
- web 2.0s
- profile links (some people also call these foundation links), which are basically accounts you create on sites like Issuu, Soundcloud, Disqus, Adobe, etc... Basically, accounts you create on high authority sites and then you edit your profile on those sites and insert a link back to your site in the profile
- article directories (although, these can also be used as tier 3 links since they're so useless nowadays)
etc..
Tiered link building is not always necessary, but it's usually employed to rank extremely competitive keywords where one layer of links won't cut it, so you need to power those links up with more authority (obtained from tier 2 which - depending on your needs - can include those weak links I've mentioned in tier 2, or can very well include another batch of quality links that you find in tier 1)