What's considerd a low grade fever? And what's considerd a high fever in adults?

A temp of. 104 or greater would warrant medical attention. Temp >107.6 °F (42 °C) creates serious risk for brain damage. What constitutes low fever is adult is debatable. Check this med line plus article: http://www.nlm.nih.gov/medlineplus/ency/article/003090.htm.
Fever. Generally, low-grade fevers range from about 100 F-101 F while high-grade fevers range from about 103 F-104 F. This is for adults. Your own "normal" (if it differs from 98.6 F might make it a bit different for you. Best!
No absolutes, 98.6 F. is >usual. Women typically run low 97's AM, upper 97's PM, men typically ~1/2 degree higher. Thus 98.6 is >normal & above this considered a fever. Generally mild fever up to the 101 range, 103 & above somewhat high oral for adults. However, temp. is under control of brain, affected by the environment, core temps (stomach, bladder) more reliable & ~1 degree higher. Oral & especially skin ?reliable.