Local Temp Table
A local temporary table, #temptable, exists only for the duration of a user session or the procedure that created the temporary table. When the user logs off or when the procedure that created the table completes, the local temporary table is lost. Multiple users can't share a local temporary table because it is local to one user session. You also can't grant or revoke permissions on the local temporary table.
Global Temp Table
A global temporary table, ##temptable, also exists for the duration of a user session or the procedure that created the table. When the last user session that references the table disconnects, the global temporary table is lost. However, multiple users can access a global temporary table; in fact, all other database users can access it. But you can't grant or revoke permissions on the global temporary table because it's always available to everyone.