A page is the fundamental unit to store data in SQL Server.
The size of a page is 8 KB. The first 96 bytes are used to store page number, page type, amount of free space in the page and the object id that owns the page.
The maximum size of a row in a page is 8060 bytes. Columns of rows that need more space than that are moved to the ROW_OVERFLOW_DATA allocation unit.
SQL Server tries to cache the content of pages in memory, in the so-called buffer cache, in order to reduce the number of disk I/O requests and thus to increase the performance.
Pages on disk and memory / dirty pages
When a DML statement modifies data, the associated page is brought into the server's buffer cache (if it is not already) where the data is modified.
After the modification, the page is not immediately written to the harddisk again. This is deferred to the next checkpoint.
A page that has modifications in memory and is not written to the harddisk yet is called a dirty page.