Hot Jupiters & Giant Neptune-mass Planets
The two largest types of exoplanets are called Hot Jupiters and Giant (or Neptune-mass) Planets.
The above photo is an actual picture of Jupiter (left) and Neptune (right) in size relation. Hot Jupiter planets have similar planetary characteristics as Jupiter, but also generally orbit much closer to their parent star (some as close as 0.015 AU), so their surface temperatures are much hotter than Jupiter's. These are the most commonly detected exoplanets with currently available technologies.
A Giant Planet or Neptune-mass Planet is a large gaseous planet whose planetary composition is much like Neptune's. It is smaller than Jupiter but still much more massive than Earth, and is mostly composed mostly of gas.
Super Earths & Earth Analogs
The next category of exoplanets is called Super Earths. In the photo above, a Super Earth (center) planet is depicted with size relative to the Earth (left) and Neptune (right). The Super Earth planet, because of its size, would be more massive than Earth but far less massive than a Neptune-sized gas giant. It would also most likely be composed mostly of rock or ice, rather than gas.
The final category of exoplanets are the Earth Analogs. These would be approximately the same size as Earth, orbiting in their star's habitable zone, and would be roughly composed of the same material as Earth. While Earth sized planets have been detected, there has not been any atmospheric characterization of these smaller planets.
The estimation for the number of planets per star in our galaxy is 1.6 (as of 2012). This number is estimated using our current technology and what we already know about exoplanet distribution. As technology advances, it is expected that more planets will be discovered, not only in number, but also in diversity. If one thing can be learned from the data obtained so far, it is that the Milky Way galaxy is home to a host of diverse solar systems, and that exoplanets are likely to be found with charactersitcs both similar to Earth, and also much different than anything that we have found before.