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to glance - to look somewhere quickly and then look away.

glance at/over/toward: He glanced over his shoulder nervously.

PICTURE: http://img.photobucket.com/albums/v66/Sonic_Fan/Reaction%20Gifs/Castiel--UnsureOkEyeGlance.gif" target=_blank>

to stare - to look at someone or something very directly for a long time.

PICTURE: http://knityorkcity.com/wp-content/uploads/2012/11/jack-nicholson-stare_o_GIFSoup.com_.gif

to glimpse - to see someone or something for a moment or not completely.

PICTURE: http://media.tumblr.com/tumblr_m6tpysjH0d1qlgn9v.gif

to gaze - to look at someone or something for a long time, for example because they are attractive or interesting, or because you are thinking of something else.

PICTURE: http://static3.beanscdn.co.uk/modules/ems-v2/article/images/gaze.gif.gif