[xpi0t0s]

TWO_TASK is an environment variable. If you set TWO_TASK=foo, then "sqlplus scott/tiger" is equivalent to "sqlplus scott/tiger@foo". If TWO_TASK isn't set, then "sqlplus scott/tiger" will connect to the local database, and throw an error if there isn't one. That's all there is to it; it means you can do a TNS connection while thinking you're doing a local connection. You can think of it as a "default database alias" if you like, but probably best to think of it as what it is.

[9648580081]

thnks interesting
could you please tell me all the default username and password in SQL

[xpi0t0s]

There isn't one. SQL is just a language; see http://en.wikipedia.org/wiki/SQL

Often Microsoft SQL Server is abbreviated to just SQL; have you done the same?