O Guia Definitivo do Yii 1.1

The required name parameter specifies a very brief description of the migration (e.g.
create_news_table). As we will show in the following, the name parameter is used as part
of a PHP class name. Therefore, it should only contain letters, digits and/or underscore
characters.
yiic migrate create create_news_table
The above command will create under the protected/migrations directory a new file
namedm101129_185401_create_news_table.php which contains the following initial code:
class m101129_185401_create_news_table extends CDbMigration
{
public function up()
{
}
}
/*
public function down()
{
}
*/
Notice that the class name is the same as the file name which is of the pattern
m_, where refers to the UTC timestamp (in the format
of yymmdd_hhmmss) when the migration is created, and is taken from the
command's name parameter.
The up() method should contain the code implementing the actual database migration,
while the down()method may contain the code reverting what is done in up().
Sometimes, it is impossible to implement down(). For example, if we delete table rows in
up(), we will not be able to recover them in down(). In this case, the migration is called
irreversible, meaning we cannot roll back to a previous state of the database.
As an example, let's show the migration about creating a news table.