All shields seen in ANH deflected turbolaser/laser/blaster fire when fired upon, but were not seen to deflect missiles or actually stop ships from flying through.
The Millenium Falcon has deflector shields, which may be angled in anticipation of attacks, and may fail against repeated battery. It is seen protecting the Falcon against turbolaser fire from Imperial Star Destroyers. Presumably most ships carry deflector shields; X-Wing and Y-Wing fighters' deflectors have distinct front and rear systems. TIE fighters are usually destroyed when directly hit, leading to the suggestion that they have weak or nonexistant shields.
Shielding is not visible in ANH, although the occasional close blue flak burst may, in fact, represent a shield impact, rather than the flak bursts mentioned in the novel.
The Death Star is "heavily shielded." Fighters flew up to the Death Star with little to no difficulty, aside from some buffeting within its "magnetic field." When tractoring the Falcon inside, they opened a "magnetic field"; presumably, and from the context of previous uses of "magnetic field" in print , this is a repulsorfield of some sort.
The ventilation shaft attacked on the Death Star was ray-shielded, a type of shielding that stops turbolasers but not proton torpedos. We may assume, judging from the Death Star's presumed invulnerability , that there were larger ray shields or deflector shields around the Death Star preventing larger ships from firing upon it. There is no skintight shield around the main portions of the Death Star, as evinced by the explosive results of Rebel fighter fire made upon the surface; however, it may simply be the case that the Death Star's armor is expected to hold up long enough for its weapons turrets to destroy any attacking ship.
At least one edition of the script, interestingly enough, refers to Luke's landspeeder as floating on a magnetic field. It is reasonable to conclude from his uses of the term that by magnetic field, Lucas is referring to some field that is repulsive.
The Death Star clearly was not physically invulnerable to attack. Attacked directly by X-Wings, the Death Star took minor but very real physical damage.
This tower exploded shortly after being fired upon by incoming X-Wings. Other attacks by the X-Wings on the surface of the Death Star produced sparks and cosmetic damage, but the occasional explosive success of strafing runs in penetrating the armor of the Death Star and blowing up portions of it suggests that a greater number of fighters, or more powerful vessels striking from close proximity, could strip the Death Star of its defensive guns in a great hurry and eventually disable the station itself.
That attack with an external fleet is not suggested as part of a feasible means of taking out the Death Star suggests either that ray shield or deflector shield surrounds the Death Star, preventing long range turbolaser bombardment, or that the Rebels cannot supply any fleet of note.