The practice of making your website search-engine compliant via

Keyword research using Google search, Wordtracker.com and Googletrends - the key is to find highly searched-for phrases with as few competing web pages as possible

Create a meaningful URL - don't just use your company name     

Standards-based meta tags

Meaningful UNIQUE content

Standards-based code with abstracted CSS and Java

Intuitive, hierarchical structure

Optimised Graphics

Optimised links

Standards-compliant code

Performance-optimised code

Any SPAM elements within your website will be penalised.  For example keyword stuffing, keyword cloaking (same colour text as background), 

Google Caffeine now looks at overall site quality when making a rank assessment and not just a few particular aspects on which a website might previously have been very strong.