isql is a utility to connect to Sybase and run SQL queries.
Right question would be difference between SQL and T-SQL.
SQL is generally referred as ANSI standard SQL which is common between Sybase, Oracle, DB2, etc. Every RDBMS has it's own advanced versions of SQL which sits on top of ANSI standards and help us to do better programming, like writting stored procedures, triggers, enhanced SQLs, etc.
Sybase has T-SQL, Oracle has PPL/SQL and I am sure there is one for Microsoft SQL Server & IBM DB2.