Is this that you were looking for?This is what you are looking for.
首先第一个是个问句.还原成陈述句：
this is something that you were looking for.可以看出是对主句的宾语提问.同时主句的宾语是从句中作宾语.对something 提问就成了：is this that you were looking for?
第二句中是陈述句.主句中少宾语.必须要用what.同时作主句和定语从句中的宾语.