What do 'sigma' and 'pi' in front of ligands in the formula of a coordination compound mean?
Concise Inorganic Chemistry1 has the following to say about this tetra(cyclopentadienyl) titanium complex:


  Tetra(Cyclopentadienyl) compounds such as $\ce{Ti(C5H5)4}$ may be made from $\ce{TiCl4}$ and $\ce{Na(C5H5)}$. The formula may be written as $\ce{[Ti(\eta_5-C5H5)2(\eta_1-C5H5)2]}$ where two cyclopentadienyl rings are attached by 5 C-atoms ($\pi$ - bonded) and two rings are attached by one C-atom ($\sigma$ - bonded). 
  
  NMR studies on these tetra(cyclopentadienyl) compounds suggest that in the $\eta_1$ rings the C bonded to Ti continually changes, and the $\eta_1$ and $\eta_5$ rings change their roles.


The first paragraph explains quite well what the author intends by the quantifiers $\sigma$ and $\pi$. Two rings are bonded by $\pi$ donation with a hapticity of 5 and two rings are $\sigma$ bonded with a hapticity of 1. This picture should clear things up



References:


Lee, J. D. Concise Inorganic Chemistry. 5th ed, Wiley publishers &amp; Blackwell Science, pp. 696, 1996