Create log file

I'm not very experienced with SQL, so hope this isn't too basic. I have a SQL database, MS SQL Server 2005 Express format. The file has an extension of MDF (say MyDatabase.mdf) and there is another file called MyDatabase_log.mdf.
What is the purpose of the LOG? Can I delete it? If I do delete it, can I re-create it?
Thanks, Alain
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