Most salsa recipes contain a mixture of low-acid foods, such as onions and chiles. Acid, such as vinegar or lemon juice must be added to prevent the bacteria, Clostridium botulinum, from growing. This bacteria produces a deadly toxin that can cause serious damage to the central nervous system or death when eaten in even small amounts. These salsa recipes have been tested to ensure that they contain enough acid to be processed safely in a boiling water canner.
The site quoted includes recipes and includes how to process in a boiling water canner.