Both tour types will include accommodations, bikes (whether they're included or rent-able) and luggage transfer.
On self-guided bike tours, you're provided with maps and directions, but you're free to cycle and schedule each day as you wish. They cost less than guided bicycle tours but still maintain a level of basic support and convenience.
On a guided group bike tour you'll travel with a group and a guide for the duration of the tour. The guide provides local insight and a structured daily schedule.
It's important to consider your comfort level navigating a different culture and a foreign language, reading maps and signs, ordering food, and dealing with flats or other minor emergencies. Some travelers don't enjoy doing these experiences, but others find them the best part of an adventure.
Here's an at-a-glance comparison of the key differences: