SOG and SCROG are the most common growing methods. I will explain the differences below.
SOG: Sea Of Green
When applying SOG, the cannabis plants are very close to each other and the room is filled with a lot of small plants. This way the yield per plant is lower but it is made up for by the amount of plants. The growth period is also shorter in this method, a major advantage if the plants do not do very well.
SCROG: SCReen Of Green
In SCROG the plants are topped, causing them to grow more in width than in height. The plants are topped by removing the top, out of which new leaves and side branches keep growing. Other side branches will grow stronger, resulting in many side branches growing in width. This way you can achieve good yields with a relatively small amount of plants but it costs more labour and time. In order to achieve the best possible result, a net is suspended over the plants. When the side branches develop and grow through the net, they can be topped at equal height. This way all tops get an optimal amount of light and you get a "screen of green". You can also preventively top your cannabis plants if you notice that they are getting too big.