Bioinformatics is a branch of biology which implements the use of computers to help answer biological questions. Classical biology involves the use of experiments, where a hypothesis is formed and objectively tested. Bioinformaticians, too, perform experiments but they may involve building computational models or using analyses to derive useful information from raw data. The possibilities of using computers with biology are vast: any biological problem can involve the use of computers.
However when one mentions bioinformatics, some applications pop up more often than others. These might include sequence alignments, searching for sequence patterns and other work with DNA, RNA or protein sequence data. This is because of the increasing amount of genome sequencing projects being completed and the usefulness that computers have brought to the analysis of genomic sequences.
Bioinformatics is a growing discipline and as computers get more sophisticated, it will be a very important complement to experimental biology.