The M&P line of double action revolvers probably were the back bone of the Smith & Wesson company. These are well made guns, built on double action K frames, with both tapered or untappered barrels, fixed sights, and a front under barrel lug which locks up the ejector rod. The gun most often encountered is the Military and Police M-10 in .38 Special. The Model 10 came in all sorts of barrel lengths from 2"-6". The 5" Victory model in .38 S&W fit the M&P profile. Purists may resent me refering to the Victory as an M&P, but all aspects of the gun fit the designation. Other M&P models included the M-13 .357 Magnum with a 4" untapered barrel, the M&P Airweight was the M-12, and there was a 9mm version called the M-547. Stainless versions of the M-10 came out as the M-64, and of the M-13 the M-65. In the '70's a short lived big bore version of the M&P came out, the M-58 a 4" bull barreled .41 Magnum.
Another S&W product which may fit the M&P line is the M-1917 in .45 ACP and, I think, in .455 Webly. I doubt if this gun was a K-frame, but it has the same outward appearance as the other M&P models.
These guns can be found used for about $150.00 for a M-10, you would have to check with a dealer for a new price. If you buy used, some of the ex-police guns may of suffered some abuse. Check for bent ejector rods, sprung cranes, out of time cylinders, firing pin indents around the chambers, and soft springs. Generally the outward appearance of the gun will tell you much about how it was cared for, and if you are not knowledgasble about these guns I would stay clear of any which appear to of been modified - such as cut away trigger guards orbobbed hammers.