Whether highlights brighten gradually or turn white at once is often what gives a digital photo away first.
Highlight roll-off is how the brightest parts of a photo approach pure white. Skies, windows, the shine on a cheek and white shirts are where you can judge it.
A schematic, not drawn from any particular film or camera. Dashed: a digital sensor, rising in a straight line until it saturates. Solid: a negative film's characteristic curve, flattening gradually towards the highlights.
Why negative film is forgiving
A colour negative film's characteristic curve has a long shoulder. Overexpose by a few stops and the highlight detail is often still on the negative, compressed together, and can be brought back in scanning or printing. That is why skies in negative-film photos often brighten gradually, with colour still at the edges of the clouds. Reversal (slide) film has much less latitude and loses highlights much more abruptly, so this is not true of all film.
Why digital highlights are "hard"
Each pixel on a sensor can collect only so much charge. At that limit more light records nothing more, and brightness is cut off. Worse, the red, green and blue channels do not clip together: when one reaches the limit first, colour shifts, so skin highlights go yellow and the bright part of a blue sky goes cyan. The edge of a clipped area is often a visible line, one of the clearest signs of a digital photo.
When you shoot
Expose for the highlights. With digital it is better to be slightly under: shadows can be lifted, clipped highlights cannot be recovered.
Use the per-channel histogram or the highlight warning. The overall histogram can look fine while one channel has already clipped.
Choose the light. Side and back light, overcast days and window light give soft highlights to begin with; reflections in direct midday sun are the hardest.
When you edit
Shoot raw. A clipped highlight in a JPEG holds no information.
Bend the top right of the tone curve into a gentle shoulder, so the brightest tones approach white gradually, rather than darkening the whole picture.
Don't overdo highlight recovery. Pushed too far, bright areas turn grey and muddy and look less like film, not more.
Where an area has clipped, lowering the saturation around it a little makes the colour shift less obvious.
One of FilmLook's readings is about highlights, and it reports roughly how much of the photo is clipped. A low reading there usually means editing can do little, and exposure on your next shoot will help more.