from BOL

sql_statement

Is the trigger condition(s) and action(s). Trigger conditions specify additional criteria that determine whether the attempted DELETE, INSERT, or UPDATE statements cause the trigger action(s) to be carried out.

The trigger actions specified in the Transact-SQL statements go into effect when the DELETE, INSERT, or UPDATE operation is attempted.

from BOL

IF UPDATE (column)

Tests for an INSERT or UPDATE action to a specified column and is not used with DELETE operations. More than one column can be specified. Because the table name is specified in the ON clause, do not include the table name before the column name in an IF UPDATE clause. To test for an INSERT or UPDATE action for more than one column, specify a separate UPDATE(column) clause following the first one. IF UPDATE will return the TRUE value in INSERT actions because the columns have either explicit values or implicit (NULL) values inserted.

for details .See this link http://msdn.microsoft.com/en-us/library/aa258254(v=sql.80).aspx