FAQ : What is the difference between Lazywriter and Checkpoint in SQL Server

Lazywriter
The lazywriter thread sleeps for a specific interval of time. When it is restarted, it examines the size of the free buffer list. If the free buffer list is below a certain point, dependent on the size of the cache, the lazywriter thread scans the buffer cache to reclaim unused pages. It then writes dirty pages that have a reference count of 0. On the Windows 2000, Windows Server 2003, and Windows XP operating systems, most of the work populating the free buffer list and writing dirty pages is performed by the individual threads. The lazywriter thread typically has little to do.

Checkpoint
The checkpoint process also scans the buffer cache periodically and writes any dirty data pages to disk. The difference is that the checkpoint process does not put the buffer page back on the free list. The purpose of the checkpoint process is to minimize the number of dirty pages in memory to reduce the length of a recovery if the server fails. Its purpose is not to populate the free buffer list. Checkpoints typically find few dirty pages to write to disk, because most dirty pages are written to disk by the worker threads or the lazywriter thread in the period between two checkpoints

Refer
http://msdn.microsoft.com/en-us/library/aa175260(SQL.80).aspx
BOL
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