What is a Class 2 Transformer
Class 2 refers to a transformer that delivers 100VA or less with a maximum output voltage of \$30\text{V}_\text{AC}\text{ }\$ or less.  Class 2 transformers are limited, either by inherent winding impedance or external fusing, to power levels considered safe enough to receive special consideration in safety requirement compliance.

They are covered by standard UL 5085-3, for which UL will be glad to take your money (link is summary only).