Individualized describes something that has been made to fit one particular person's needs, rather than handed out the same way to everybody. It is the adjective form of the verb individualize, and honestly you will see the adjective far more often than the verb. It lives in a small number of set phrases you should simply learn whole: an individualized learning plan, individualized instruction, individualized attention, an individualized treatment plan. In American schools there is even a legal document called an IEP — an Individualized Education Program — for students who need extra support. The word carries a promise of care: it says 'we looked at this one person and built this for them'. That is why schools, clinics, and gyms like it in their advertising, and why you should read it with mild suspicion when it appears in an advertisement, since anything can be called individualized. Its close relatives are personalized (which now often just means your name was printed on it) and customized (which suggests choosing from options, like a car). Individualized is the most serious-sounding of the three and the one that suggests a professional judged what you needed. British writing uses individualised.