The use of grounding symbols does not always strictly adhere to industry standards; it can be difficult to understand which ones are used and in what context. Most of the time the ground symbol you are most familiar with, the 3 decreasing-length lines, is the one that is used in a generic sense to illustrate ground.
The three diagonal lines and the decreasing horizontal bars represent chassis and earth ground, respectively. Furthermore, the three decreasing-length parallel lines represent analog or circuit ground. The hollow triangle usually means digital ground, but is often used as a reference ground. A letter or number in the triangle may be used to indicate references that are in common with each other.
Note that none of these grounds are necessarily equivalent. Ground is a point of reference from which a potential (voltage) may be measured. Different grounds will have voltages between them that we must take into account when connecting equipment together and making measurements.
Earth ground means that the earth itself is used as a reference point.