Difference between NE555 and NE555xx
The difference is in the packages. The function is the same. You can search for package types on TI page, they have nice documentation. Datasheet

As you can see, in NE555xx, the xx represent a different package type. There is more information about these xx on pages 20-22 inside the datasheet. The letters NE/SA/NA/SE before the 555 represent slightly different manufacturing processes &amp; part QA, corresponding mostly to safe operating temperature ranges (commercial, industrial, automotive, military) &amp; voltages. You need to  read the datasheet and select the chip which is appropriate for you.