An autotransformer is a transformer in which the primary (input) and the secondary (output) are electrically connected to each other.
An isolation transformer, also known as an insulating transformer, has complete electrical separation between the primary (input) and the secondary (output).
This is illustrated in the drawing below:
An autotransformer changes or transforms only a portion of the electrical energy it transmits. The rest of the electrical energy flows directly through the electrical connections between the primary and secondary. An isolation transformer (insulating transformer) changes or transforms all of the electrical energy it transmits. Consequently, an autotransformer is smaller, lighter in weight, and less costly than a comparable kVA size insulating transformer.
Please refer to this question for additional information on autotransformers.
Buck-boost transformers are frequently field-connected as auto transformers.