English for Special Purposes or ESP is subset of English for a specific purpose which is likely related to their work.
For example, a doctor who wishes to work in an English speaking country will need ESP biased towards medical English; a footballer from Italy signed to an English club will need English related to that sport.
ESP courses tend to be very specific and tailored to suit each student.
ESP can be general in nature (English for Hotel Employees, for example, who deal with the public) or highly specific and related to a single task (English for writing CV/Resumes, for example).
One problem for teachers is the actual content of an ESP course. You might be teaching Medical English one day, IT English the next and English for Archaeologists on the following day. Preparation is always needed when teaching English, but with ESP it is vital that you are aware of the specifics of the language you are teaching in terms of vocabulary and need.
One issue with ESP is the level of the learners. Students usually need to have at least a basic grasp, if not higher, of English before dealing with the ESP aspect of the language. In other words, ESP is an adjunct to a good general knowledge of English.
Any decent teacher should be able to handle an ESP class. Aside from the standard English which will be integral to the class, the teacher will need to know specific vocabulary for the subject and the basic concepts to be explored by the class. In many cases this can be covered by reading a book or two but a very useful aid is having the students provide this information which will not only help the teacher but also help them.
Of course these days with internet it is relatively easy to find and download specialised knowledge!
Because coursebooks tend to be very generic, ESP teachers often find themselves having to write their own materials for a class. Sometimes these can be adapted from existing general materials, but more often they will be specifically tailored to a particular problem a student might have or needs to practice.