Tactile - browns, clears, blues, etc. - have a slight "bump" in the force required to keep pressing the switch down at or near the actuation point. Imagine it as the straight line equivalent of a panel mount toggle switch, where you get that nice crisp "snap" and change in feel when the contacts actually make. That's kind of what a blue feels like, but obviously being a keyboard switch not as much force is required. A blue or brown feels quite light to someone making the switch from rubber domes usually. A brown has the same kind of feel, but not as crisp, as a blue. and is quiet save for if you bottom out the key. (the marketing of browns as "silent" is not completely accurate however - you still get a little noise from a browns board, a lot of that being from when the keys return to the top of their travel. That said, for someone with a light touch a board with browns or clears is still probably quiet enough to not be offensive in an open office environment.)
Linear - blacks, reds - do not have that "bump" at all. The force on the keycap increases linearly with the distance that you have pressed it down, and there is no sound or change in feel to let you know that you have actuated the switch. However, they do feel smoother than most of the tactile switches out of the box.
