A parser is the person, or much more often the computer program, that parses. The job is the same one you meet under parse: work out the grammar of a word in a sentence, or the grammar of the whole sentence, by naming what each piece is doing. In an old grammar class, the parser might have been the student who stood up and did that by hand. Though actually, almost every time you see parser now, it is a machine. A parser reads a line of words or symbols and builds the structure: where a sentence starts, which bit is a name, which bit is a command. Compilers, search boxes, and language tools all use one. It is a noun. You do not “parser a sentence”; you parse it, and the parser is what does the work. In computing talk it is a completely ordinary word; in everyday talk outside that world, people rarely need it.