A short, easily understandable USGS booklet titled, “Geologic Time” by William L. Newman discusses the relative time scale, fossil succession, rocks and fossils, and radiometric time scale. This is a good introductory resource for students without much a background in the geologic time scale.
http://pubs.usgs.gov/gip/geotime/contents.html
Need mnemonic devices to help remember the order that the periods and epochs occurred? Try “Dr. Bob's Geologic Time Page.” For periods consider the first letters of each word for, “Quiet Time Can Just Tease People Particularly Men Dodging Spouses Ordering Chores” or “Cold Oysters Seldom Develop Many Pearls, Their Juices Congeal To Quickly”