Search Engine ranking depends on more than 128 factors that can be classified into two groups :

a> On page Optimization
b> Off page Optimisation

The most important criteria of achieving high rankings is to have a scientifically designed and programmed website with rich content. Content is gold in SEO industry.

Following are some of the factors that influence search engine rankings :
ON PAGE FACTORS :

Keyword Use in Title Tag
Keyword Use in Body Text
Relationship of Body Text Content to Keywords (Topic Analysis)
Effective Use of header tags - h1,h2,h3
Keyword Use in Domain Name
Keyword Use in Page URL
Keyword Use in Alt Tags and Image Titles
Keyword Use in Bold/Strong Tags
Keyword Use in Meta Description Tag
Keyword Use in Meta Keywords Tag

OFF PAGE FACTORS

Global Link Popularity of Site
Global Link Popularity of All Linking Sites
Age of Site
Anchor Text of Inbound Link
Topical Relationship of Linking Page
Age of Link
Domain Extension of Linking Site (edu, gov, com, ca, co.uk, etc)
Topical Relationship of Linking Site
Text Surrounding the Link
Internal Link Popularity of Linking Page within Host Site/Domain
Temporal Link Attributes (when in time the link was created/updated)
PageRank (as measured by the GG Toolbar) of Linking Page

Link Popularity of Site in Topical Community
Topical Relevance of Inbound Links to Site
Link Popularity of Site in Topical Community
Rate of New Inbound Links to Site
Relevance of Site’s Primary Subject Matter to Query
Historical Performance of Site as Measured by Time Spent on Page, Clickthroughs from SERPs, Direct Visits, Bookmarks, etc.
Manual Authority/Weight Given to Site by Google
TLD Extension of Site (edu, gov, us, ca, com, etc)
Rate of New Pages Added to Site
Number of Queries for Site/Domain over Time

Link Popularity within the Site’s Internal Link Structure
Quality/Relevance of Links to External Sites/Pages
Age of Document
Amount of Indexable Text Content
Quality of the Document Content (as measured algorithmically)
Organization/Hierarchy of Document Flow (i.e. broad > narrow)
Frequency of Updates to Page
Number of Trailing Slashes (/) in URL
Accuracy of Spelling & Grammar
HTML Validation of Document (to W3C Standards)