HanShi mentioned local and global temp tables. You don't see this as often but a global temp table has two numbers signs in front of it like so: ##temptable.

To get some understanding about the difference between the two, open a new query window in SSMS and run these SELECT statements:

CREATE TABLE #LocalTempTable(xId int);--(1) Local Temp Table

CREATE TABLE ##GlobalTempTable(xId int);--(2) Global Temp Table

INSERT INTO #LocalTempTableVALUES(1);

INSERT INTO ##GlobalTempTableVALUES(1);

Then, in the same query window, you could successfully run these queries:

SELECT * FROM ##GlobalTempTable

SELECT * FROM #LocalTempTable

If you open a new query window, however, and run the two SELECT statements above, the first will be successful but the second one will fail.