There are many types of engagement rings, the main attribute of an engagement ring is that at list one of the stones is a diamond stone (the main stone), as regard to the shape, cut and carat, each ring has a distinct and different features.
Take for instance the classic solitaire engagement ring, it usually feature a round solitaire diamond which reflect light in a manner that gives the best visual affect.
Once you decide the type of stone, you need to decide the style of setting, Solitaire is a style were there is one central stone piece, another style is the trinity ring style which is three stones with the central stone larger then the rest.
The stone itself can come in different shapes and set differently in the ring, you can have a round stone which is the most popular or Marquise emerald or pear.
Most diamond rings are set in gold or platinum.
It is preferable when buying a gold band to buy 18k although 14k can also be part of a lovely quality ring.
Once you decide which shape you want then you need to consider the quality of the stone you want, there are different stones with different quality, you need to understand the 4 C's that relate to the quality of the stone, and then before you buy it is a good idea to ask for a certificate that ensures the diamond was tested in a lab and the carat, cut, color and clarity are correct.