A soft-shell jacket is usually water- and wind-resistant, but not as weatherproof as a hard shell. What you sacrifice in weatherproofing, though, you make up with improved breathability that helps keep you dry during high-output activities. (Think hiking up a long, steep hill.) Soft-shell jackets usually have some stretch to the fabric too, making it easy to get a slim fit but still do everything from rigging your camp hammock to scrambling up a steep slope.
Soft-shell jackets are usually heavier and less packable than hard shells too -- but they're also more comfortable against your skin, and can function as a mid (insulating) layer beneath a hard shell.
What kind of jacket should I buy?
Soft-shell technology is improving, but they're still not as weatherproof as the best hard-shell jackets -- so soft shells are usually best for use high-exertion hikes in relatively mild conditions. If you ski or run during the winter, a soft shell can cross over nicely from hiking to those other sports.
On the other hand if you know you're going to be facing prolonged wind and rain, or if your budget only stretches enough to purchase one jacket, I recommend going for a good hard shell -- it is ultimately more versatile, and offers better protection against bad weather when you really need it.
As with hard-shell jackets, your soft shell's waterproofing and breathability are usually listed on the tags. Waterproofing is standardized across the industry; the higher the number, the better. Breathability measurements aren't standardized, but in general a higher number is more desirable (just take those figures with a grain of salt).
To recap, here are the key differences to be aware of between soft-shell and hard-shell jackets:
- Every hard-shell jacket has a hood; not all soft-shell jackets do.
- Soft shells are typically more breathable than hard shells.
- Soft shells have more stretch than hard shells.
- Soft shells feel better against your skin.
- Ventilation zips aren't as common on soft-shell jackets as on hard shells.