I cannot believe how many events I go to where no one has any idea what is appropriate to wear. I've seen Hawaiian shirts at weddings, I've seen flip flops at funerals, I've seen jeans at formal dinners, I've seen workout pants at the office. If none of these things seem wrong to you, please read on, for your own sake as well as everyone else's who has to look at you.
By being alive, you have accepted the Terms & Conditions of Being A Human, which includes knowing these 7 levels of formal dress and when to wear the corresponding clothing.
Where you'll see it: Everyday life; some very casual offices where client interaction is minimal.
What the men wear: Basically whatever, most likely jeans & a shirt.
What you should wear: Basically, wear whatever you want, as long as it's appropriate (no short skirts, low-cut tops, or anything that might make people mistake you for a hooker).
2. Business Casual.
Where you'll see it: Some offices such as marketing + design (almost all offices will specify the level of dress when you're hired); work functions outside of working hours.
What the men wear: Dress pants and a casual shirt; jeans & a dress shirt; tie optional; full suit not required. If you are a man, read this article.
What you should wear: I can't possibly list all of the possibilities for business casual. Basically, it's halfway between Casual and Business Formal. This means, to list off a few: skirt + blouse, flat shoes or high heels, nice jeans, cardigans + sweaters, dresses, dress pants. Denim is allowed, as long as the opposite piece is dressier - for example, jeans with a blouse, or a chambray shirt with dress pants.
Business casual allows for you to have more fun with your outfits - statement jewelry, more patterns + layering, more colours - it's all permitted. In fact, it's encouraged. For a few outfit ideas, check out this article.
What you should not wear: Tights as pants, flip flops, jeans with rips/tears/overdone distressing, low-cut tops, too-short dresses or skirts, enormously baggy shirts/sweaters, any t-shirt you might see someone wearing at the gym or the club, anything that could possibly be misconstrued as workout wear OR pajamas.
3. Business Formal.
Where you'll see it: Typically the oil/gas industry, legal industry, financial industry (the more Serious ones).
What the men wear: Suits.
What you should wear: Dress shirt + dress pants, blazers, blouses, high heels, dresses that are more on the conservative side. Keep it tailored, clean, professional, and relatively buttoned-up.
What you should not wear: Neon colours, open-toed shoes, flip flops (actually, just sandals of any kind), skirts more than a couple inches above the knee, tights as pants, enormously baggy shirts/sweaters, gaudy jewelry, low-cut tops, anything that even DREAMT about being workout wear or pajamas, denim.
* Note: At an office that is Business Formal, "Casual Fridays" do NOT include jeans, unless otherwise specified.
4. Semi Formal.
Where you'll see it: Most weddings, engagement parties, bridal showers, some galas, charity events, fancy birthdays or anniversary parties, etc. Usually specified.
What the men wear: Suits, dress shirt + dress pants + tie.
What you should wear: Similar to business casual, but a teensy bit dressier and a bit more feminine. Cocktail dresses, high heels, floor-length dresses (not ball gowns, not in fancy fabrics), clutch or small purse, skirt + blouse separates ONLY if they're both very dressy and paired with heels.
Where you'll see it: Some weddings, every funeral, high-end dinner events, etc. If formal attire is required, it is almost always specified, so keep an eye out.
What the men wear: Suits.
What you should wear: Floor-length dresses (no cotton or jersey), a cocktail dress that's in a formal material, high heels, nice jewelry, clutch or small purse. You probably don't own anything that's that formal right now (most people don't), so if you get invited to a formal event, be prepared to buy something.
and just a few bonus...
6. Black Tie.
Black & White Tie events will hardly ever come up in most people's lives, but I've included them because they are important to know about. Just in case!
Where you'll see it: Galas, high-budget charity events, some weddings,
What the men wear: Tuxedos.
What you should wear: Knee length or longer cocktail dress, long gowns, satin/taffeta and all of those fancy fabrics, your nice jewels that you never get to wear, high heels.
What you should not wear: Anything not listed above. Sorry.
7. White Tie / Ultra-Formal.
Where you'll see it: Incredibly fancy galas.
What the men wear: A specific shirt, white tie, and jacket with tails.
What you should wear: Long gowns aka ball gowns, high heels, and the nicest jewels you own.
And when in doubt, everyone... GOOGLE IT!