Dump all databases for backup:

mysqldump -u root -p[password] --opt > [/tmp/alldatabases.sql]

Dump one database for backup:

mysqldump -u [username] -p[password] --databases [databasename] > [/tmp/databasename.sql]

Dump a table from a database:

mysqldump -c -u [username] -p[password] [databasename] [tablename] > [/tmp/databasename.tablename.sql]