Shear, poll, clip, trim, prune, lop, snip, crop are comparable when they mean to cut off something (as a piece, an excrescence, or a limb).
Shear is the most general word of this group; it usually implies the use of a sharp cutting instrument (as shears, a razor, or a sharp knife) and, as its result, a close and even or a clean cut through, or off, or away, or from something.
The term may or may not imply injury and suggests improvement more often than destruction.
Poll implies the cutting of the hair as closely as if shaved; in this sense the verb seldom takes the person as its object, but the part affected.
It sometimes refers to the cutting off of the top or head of trees (as willows), often in order to provide new growth suitable for basketry, but sometimes in order to encourage the throwing out of branches from below.
Clip suggests a cutting evenly or closely without any indication of how much or how little is cut off.
Trim (see also STABILIZE ) always implies the removal of something unwanted or overlong by or as if by cutting or clipping in order to improve the appearance of a thing, or to adjust it to something, or to prepare it for a definite use.
Prune implies a trimming of a plant (as a tree or shrub) by cutting out superfluous parts (as dead branches) not only to improve its shape but to promote its growth or bearing.
Consequently prune in broader use implies a cutting down or out or excision so as to remove useless or needless material (as in written matter).
Lop implies a cutting off or away by or as if by an axe, especially of what is superfluous; typically it suggests pruning and the removal of dead or unnecessary branches or boughs, but it may suggest the similar removal of something that may be regarded as improperly associated or as an excrescence, a nuisance, or an interference.
Snip, like clip, may imply the employment of scissors, but it may also suggest the use of sharp fingernails or of any other instrument by which a part may be pinched or cut off; it differs from clip in emphasizing suddenness and quickness in movement. It, therefore, often suggests a cutting off of a small piece at a time or a cutting into bits.
Crop, in most of its meanings, implies the cutting off of the top (as of a tree or grass), but when it emphasizes that implication, it usually suggests the cutting off of a piece at the top (as for identification or punishment) or a cutting extremely close (as of the hair).