So "to see" is the action of looking at something, to see something happening or just to see anything, etc.
"See" is a verb. So you use it when you look at something or want someone to see something;
Ex: "I see myself in front of the mirror"
Ex: "Can you see me from there?"
Ex: "See, that's what I told you about!"
To be seen means that someone looks at something, sees something.
"I've seen myself in front of the mirror for over 2 hours"
"Have you seen me from there?"
You can also make it as a passive voice.
Active voice: "Ann saw John near that tree last night"
Passive voice: "John was seen by Ann near that tree last night"
Another example: Active voice: "That man saw the murderer"
Passive voice: "The murderer was seen by that man"
See is a verb, sees is the present tense of this verb and seen is the past tense of the verb.
Hope this helps.