Rosemary, speaks of Arbaviatea, which fall into two basic categories, Green Arborvitae, and Giant Arborvitae. They are Cedar trees (essentially), and the Green Arborvitae, grows to a mature height of 15'-18', where as the Giant Arborvitae, can grow to a mature height of 30' and beyond.
Here is a mass of Emerald Green Arborvitae. They have a standard growth rate that is fast, and are a superb choice for privacy/screen options, however, they do not spread or cone (cone, as in shape) like that of the Dwarf Alberta, which most closely resembles a traditional Christmas tree look.
Plus side of Arborvitae varieties, they are very affordable, extremely hardy, and are rich in colour year round.
Negative side of Arborvitae, deer will feed on them (winter months), thus ruining the natural esthetically pleasing shape and appearance of them, so wiring or wrapping will be required if you have deer that frequent your area. If no deer, you have nothing to worry about!