Generally speaking, the terms "placement" and "internship" have the same meaning - a period of pre-career full-time work experience at a company a person has an interest in working for, held during or just after an undergraduate or post-graduate degree course. However, the terms do have a subtle difference in meaning and each generally refers to work experience held in specific types of industry.
The term "internship" is usually used by city firms such as accountants, investment banks and management consultancies. An "internship" usually last from a week to 12 weeks and may involve some travel. "Internships" are usually offered to students and graduates outside of university term time (placements are usually held during university term time). "Internships" are usually focussed on learning almost as much as working, although internships at some city firms (such as investment banks) usually also involve a great deal of tea and coffee making and errand running, for more senior graduate trainees!
The term "placement" is usually used to refer to long or extended periods of internship held by students during a four-year university degree course, that has been specifically designed to accommodate a year "in industry". Four year degree courses with a vocational element are becoming increasingly popular and consequently more and more employers are now offering "placements", including accountancy firms.
Both placements and internships are great features for your CV. In fact, any experience of working within an organisation in the industry you wish to work in is incredibly worthwhile and is a great indicator that you have career motivation and strong commercial awareness.