"Background" doesn't have to mean "boring": The background dress's simplicity means it's a blank canvas, and you can go wild with different colour schemes and accessories for a different mood each time. You can play with eras (with dainty gloves and a snood that shirtwaist is 40s; switch those out for a wide-brimmed floppy hat and knee high boots, and it becomes 70s) and adapt it to a variety of occasions (changing your shoes and hairstyle take a swing dress from casual to cocktail).
When it comes to prints, polka dots, ginghams and plaids (choose monochromatics - which isn't to say neutral - for maximum versatility) are always very wearable. Great novelty print dresses can often be expensive, so it's usually best to be more sure of your style before you start investing. Figure out what styles work on your body shape: A 1940s or 50s shirtwaist is generally a safe bet for most figures; on the other hand you may find that (for example) dropwaist styles, shift dresses, high necklines or puff sleeves do nothing for you, so you'll know to avoid them.
When it first appeared in the late Victorian era, a shirtwaist was a type of women's blouse ("waist" meant bodice or blouse) inspired by men's tailoring and free from the elaborate ornamentation that was common at the time. Gradually the term came to mean a one-piece dress with a shirt-like, collared bodice. Shirtwaist dresses can button from neck to waist seam, or through to the hem. Traditionally they are daywear, though 1950s versions in silk and acetate taffetas made the transition to cocktail wear.
As you can imagine, this style was originally conceived as workwear, essentially an apron intended to protect the frock underneath. By the 1930s it had evolved into a fashion garment in its own right. It was popular during the 1940s (it was a perfect "make do and mend" project) and through the 50s in both slim and full skirted styles. The pinafore had a complete makeover in the late 60s and early 70s at the hands of designers such as Mary Quant, who created A-line, mini length versions and daring cutaway variations. Whichever era you choose, a pinafore/jumper dress is great daywear, which will take you from early autumn through to next spring just by changing what you wear with it (blouse, cashmere sweater, cardigan). Mine (made from the 1950s pattern above, but I drafted a new neckline) is a wardrobe staple. New Look currently has a perfect 1960s style mini pinafore which is seriously tempting me to introduce some of the 60s into my wardrobe.
The fashion peplum has its origins way back, as a detachable flounce intended to give a little extra emphasis to one's bustle. Some flapper frocks featured similar flounces to their dropwaist dresses in the 1920s to add swish to their charleston, and peplum styles continued into the 30s. The 1940s saw a real heyday for peplum dresses exaggerating the nipped-in waist, with short peplums, long peplums, asymmetrical peplums, flounced and flat, draped and tiered peplums, peplums which tied sarong-style at the waist. In the early 1950s the peplum one-piece dress all but disappeared, though peplum suit jackets were wildly popular. Peplum dresses enjoyed a huge resurgence in the 1980s, then faded into obscurity during the 90s, only to re-emerge once more in about 2009.
Drop-waist dresses are of course most closely associated with the 1920s, but by no means did they end there. Towards the end of the 20s waistlines moved back up toward the natural waist, and remained there through the 30s, but in the mid-40s a new style of dropped waist emerged, slim fitting over the waist and hips then flaring into an A-line or gathered skirt. These fitted drop-waist styles remained popular through the 50s. Fashion started de-emphasising the waist once more in the late 50s in the boxy silhouette associated with the "Mad Men" period, paving the way for a revival of the 1920s loose-fitting drop-waist. Late 60s and early 70s mod fashions featured drop-waists as an alternative to the simple shift. The drop-waist enjoyed another resurgence in popularity during the 1980s.
A princess dress is one in which the shaping is achieved through vertical panelled construction, with no waist seam.