mysqldump -a -u USERNAME -p DATABASE > FILENAME.mysql
You will be prompted for your database password and then the DATABASE will be dumped to a plain-text file called FILENAME.mysql.
The resulting file, FILENAME.mysql, is a full backup with which you can fully restore your database in case of problems.
Restoring via the command line
1. Drop the database
mysqladmin -u USERNAME -p drop DATABASE
2. Recreate the database
mysqladmin -u USERNAME -p create DATABASE
3. Import the backup data
mysql -u USERNAME -p DATABASE < FILENAME.mysql