Recovery model backup and restore database

I want to do a backup and restore operations of my base, using SQLCMD.exe.

For that, I use backup database mybase to disk='File'

Then, I try to do a backup of transact log but like this:

backup log mybase to disk='File'

But I have this message : Cannot do a backup log on database which is a simple recovery model.

I read the BOL and I saw there are to type of recovery model: Simple and full. But I didn't understand very bien explications in BOL.

My questions are:

What is this models?

When are they used ?

ThanksRecovery model determines how the transaction log files are handled in the database. This is in turn affects performance, log file usage, backup/recovery, amount of data loss etc. Please refer to topic below for overview:

ms-help://MS.SQLCC.v9/MS.SQLSVR.v9.en/udb9/html/606e0487-0c76-4875-af44-ef763cc5a73a.htm

You should also look at the transaction log architecture topic and why it is necessary in a relational database system.