You will find it hard to get a more clear definition of a type of essay than a cause and effect one. As the name implies there are two aspects to this type of essay. You have to write about what has happened and then explain (a) what caused it and (b) what were the consequences or effects. That in a nutshell is a cause-and-effect essay.
Now you need to be aware that sometimes the cause of an event cannot be attributed to a single thing. In other words there may be several causes or many causes which have brought about the event. That is all the more reason why you should ask your teacher or tutor beforehand to precisely explain the situation. Maybe you are only required to list one cause in which case if you list several you will be doing yourself a disservice.
You are required to develop your thesis for this cause-and-effect essay and to do so you need to be very precise in your language. It is so important that you are able to pinpoint an exact and possibly a singular cause in your thesis statement. You need to be aware of the knock-on effect. Some people are able to pinpoint the cause and in fact that is just the first of many. The initial cause impacts something else which can impact something else again and so on.
You need to know the effect or effects of the cause before you write your thesis statement. Remember that this is a two-part essay where the cause and the effect have equal importance. Knowing what happens at the beginning and the consequences of that happening will help you create the ideal thesis.
The use of transitions is always important in essay writing but never more so with a cause-and-effect essay. Just the very nature of this type of essay suggests a knock-on effect. A transition sentence is one which leads from one idea or thought to the next and hopefully it is a seamless connection. So bear that in mind in the writing of your essay.
You also need to understand the chronological sequence of events. Remember that the person assessing your essay will be going on a journey. This is not the time for flashbacks or a back story but rather to tell in a linear fashion exactly what happened and once what happened did happen the consequences that followed.