Simply speaking, computational chemistryis a branch of chemistry that uses the concepts of theoretical chemistry and takes the assistance of computers and solves problems related to chemistry. A detailed lecture series including theory of ab initio, DFT, semi emperical and Molecular Mechanics alongwith a brief guide on how to use Gaussian Softwarethrough GaussViewcan be found at the Nick Mosey's webpage. GaussView is the software used to build/view molecules whereas the actual calculations are run in Gaussian suite of programms.
There are a variety of softwares available for computational chemist. Some softwares can perform a variety of calculations where as others are specific for a particular type of Calculation. A brief list of different types of calculations softwares are:

BiomolecularModelling (Docking) for Proteins and Enzymes

Molecular Mechanics

Quantum Mechanics

Semi Emperical

Valence Bond

Molecular Desing Software
A complete listof computational softwares can be found here. Most commonly used softwares are:
1. Docking/Biomolecular
a. AutoDock
b. Gold
c. AutoDockVina
d. Flexx
2. Quantum Machenical
a. Gaussian
b. Gamess (US)&Gamess (UK)
c. Spartan
d. FireFly
e. Jaguar
f. Polyrate
g. MolPro
Gaussian Help Sources

Gaussian keywords

CCL discussion forum for computational help

Gaussian examples page

Dr. Joaquin Barroso's Blog
Focus Areas

Mechanistic understanding of chemical transformations, particularly focused on catalysis

Photoswitches

Conducting polymers

Frustrated lewis pairs

Aromaticity

Theoretical estimations of noncovalent interactions biologically important complexes.

Dyotropic rearrangement
