A summer school is a programme at a university abroad where you can subscribe for a course during the summer months. Utrecht University is not responsible for the (content of) the courses these universities offer, nor for the facilities they provide. The costs involved are at your own expense. In most cases, the credits earned will not count towards your degree at UU.
The difference with an exchange programme is that the latter is based on an agreement between two institutions or departments to exchange a certain number of students during one or two semesters during the academic year. The credits obtained during an exchange semester usually do count towards the degree in Utrecht.
On the Dutch version of this site you will find an overview (in English) of the information on summer schools that we have received from our partners. This list is not exhaustive, so feel free to look for opportunities yourself. If you are interested or have any questions, you can contact the summer school in question directly.
Another option is to check the site of Utrecht Summer School. They offer a number of summer school courses in cooperation with their partners.