This is the fourth (and probably last) type of
electron orbital. There are seven f
orbitals per shell starting with the fourth, however the orbitals are so energetically unfavourable that the 4f orbitals are above the 6s (during filling). These orbitals are very complex shapes (okay I'm wimping out). The f stands for fundamental.
These orbitals are particularly associated with the chemistry of the lanthanides and the actinides.