Mary Lorenz, CareerBuilder, says a suit is no longer the universal standard. IBM's idea of professional may not be Google's, in other words.
Startups are often looking for people. But they tend to be more lax in dress.A suit may mark you as uptight--not a good fit for a place with a bar and Foosball.
Yet, some places require suits or equivalent female attire every day--law firms or finance firms come to mind.
You need to do your homework. But here are some definitions:
Business professional. Suits for men, skirts or pants with heels for women. Men can also wear a blazer, button-down shirt, tie, dress shoes.
Business casual. For men, dress slacks or chinos, a button-down or polo shirt, belt, dress shoes. For women, a conservative one-piece dress or blouse/sweater and skirt, dress shoes or boots.
Casual. No jeans or flipflops, For men, khakis, long-sleeve shirt, belt, dress shoes. For women, a collared shirt with pants or pencil skirt, or a work dress.
Check out the company ahead of time--see how people dress on the website. Ask people who work there.
If in doubt, err on the side of overdressing a little--a step above the usual attire in the office (that's for after you get the job).
Blue is the most employer-approved color, followed by black, brown and gray.
Also--don't wear perfume, aftershave or cologne. Some hiring people have allergies and it can be a distraction.
Basically, I would say don't show the "girls," wear flipflops, sandals or ripped jeans, or throw on so much clunky jewelry you look like an outtake from Harper's Bazaar.