Page is the smallest unit of storage in SQL Server.
Like a brick is the smallest unit for a house page also the same for SQL Server.
Size of page is 8KB (8192 bytes). So a database contains 128 pages per megabyte. A page contains 3 sections mainly like Page Header, Row Offsets and data. Page header occupies 96bytes it contains information like page number, amount of free space and so on. Row Offsets stores the information about the order of rows like row offset 0 indicates its first row, 1 indicates its second row and so on. Data is where the actual information/data is stored.
So the maximum amount of data that can be stored in a Page is 8060 bytes.
Group of 8 pages is termed as an extent. So size of extent will be 64KB.
Based on type of pages an extent contains it is classified into 2 types.
If an extent contains all 8 pages belongs to same object it is termed as Uniform Extent.
If an extent contains 8 pages which belong to different object these are Mixed extents. So maximum of 8 different objects can have pages in an extent.