The following code are to be included in the header of your Tumblr theme:
{Title} – The html title of your blog

{Meta Description} – An html description of your blog for use within the meta tag
{Favicon} – A dynamically generated favicon url from your portrait photo

{RSS} – The url to the RSS feed of your tumbleblog

The following code is for your profile picture on your Tumblr:
{PortraitURL-64} – Different sizes are available (16, 24, 30, 40, 48, 64, 96, 128)

The following code is to be used where different types of posts will be displayed:
{block:Text}{/block:Text} – Displays Text posts

{block:Photo}{/block:Photo} – Displays Photo posts

{block:Photoset}{/block:Photoset} – Displays Photoset posts

{block:Quote}{/block:Quote} – Displays Quote posts

{block:Link}{/block:Link} – Displays Link posts

{block:Chat}{/block:Chat} – Displays Chat posts

{block:Audio}{/block:Audio} – Displays Audio posts

{block:Video}{/block:Video} – Displays Video posts

{block:Answer}{/block:Answer} – Displays answer posts

{Permalink} – The exact url for a single post

{ShortURL} – The sharing friendly short url for a single post

{PostID} – The unique numeric post ID for a single post

The following code is to be used for the Next/Previous links:
{block:Pagination}{/block:Pagination} – Only displays if there are previous / next pages to link to

{block:PreviousPage}{/block:PreviousPage} – Only displays if there is a previous page

{block:NextPage}{/block:NextPage} – Only displays if there is a next page

{PreviousPage} – Url for the previous page

{NextPage} – Url for the next page

{block:PermalinkPagination}{/block:PermalinkPagination} – Only displays if there are previous / next posts

{block:PreviousPost}{/block:PreviousPost} – Only displays if there is a previous post

{block:NextPost}{/block:NextPost} – Only displays if there is a next post

{PreviousPost} – Url for the previous post

{NextPost} – Url for the next post

Additional Resource: freecodesource.com