Psionic abilities generally fall into two realms
(Mind-to-mind abilities are further categorized into passive or active abilities)
Passive abilities: are those in which the psionic can only sense and cannot influence
Active abilities: are those in which the psionic can both sense AND influence
(Listed below are just a few examples of each type of ability, I won't list everything because that would take me a REALLY long time)
Passive and Active
Non-psis can be taught to shield against psionic attack and not broadcast their thoughts, and with more intensive training, completely block it.
Shielding traditionally occurs in three stages
1.) Learing to actively quiet one's own psychic signature. The mental version of quieting and controlling your breathing, in a way. Instead of broadcasting your thoughts and energy, you can internalize those processes.
2.) How to actively identify a foreign mantal presence, get a feel for the levels of potential scanning and activity on the mind.
3.) Creating the actual shields. In some cases, these might be imagined "walls", keeing out the other's thoughts, in others they can be repeated phrases or meditative mantras. For others, it be projecting certain unpleasant memories or thoughts in an effort to drive the psionic away, or a combination of several techniques.
Although, without intensive training, and sometimes even with it, a high-level psi can basically slip past those shields and alter the memories or implant suggestions, effectively wiping traces of the invasion. In some cases, it is even possible to simple find a tiny crack in the shied, allowing the psionic to slip those suggestions past a person's defences without them being aware.