Genomics is the study of gene expression, but on a more global scale. So rather than study the expression of just one gene, we study a whole set of genes, conceivably the entire genome, at some particular point in time.
Toxicology is the study of the adverse effects of exposure to chemicals.
Toxicogenomics is a hybrid or chimeric science, the child of toxicology and genomics. Bioinformatics takes on many faces, and is extremely diversein what problems it can tackle. Bioinformatics is also used in toxicology to help predict changes in gene expression.