Many day hikers have a bunch of questions about transitioning from a day out on the trail to a night or more in the wilderness. My list may be a little different from yours, but keep in mind that not everyone will be the same. This is a list of the things I put in my pack for a multiple day hike.

Tent
Sleeping pad
Sleeping bag
3 Liters of water (not necessarily full)
Water filtration device
Head lamp with extra batteries
Sunglasses
Head net for bug control
Bug spray (permethrin)
Basic first aid
Daily vitamins
Ibuprofen
Stove and gas
Spork
Dinner
Lunch
Breakfast
Snacks
One extra of each meal per day out
Toilet paper
Wet wipes
Food bag to keep out pests
Compass
Map of the area Check out this app!
Fire started for emergencies
Small knife
Any meds?
Clothing (see paragraph below)
Smart phone (see paragraph below)

Clothing - My method on the trail is to have only one extra set of clothing. I put my recently worn items in the top of my backpack and I make sure to loosely pack them so they can air out some. Then I swap them out the next day and repeat the same process. It is important to bring a warm layer including gloves in case the temperature drops. I also have a rain coat and kilt to fend off the weather, but some people bring umbrellas for hopes of being dry.

Smart phone - A smart phone is the one item that saves me a lot of space. I can leave my camera at home and I used to always bring a book too. Longer hikes do require that I bring a battery back-up though, that weighs close to the same as a book, but saves on space for sure! Note: I know that some people prefer their bigger cameras and I appreciate that, as well as the great pictures they produce. Though for me, a non-photographer, I gladly leave it behind.

Favorite hiking app - The newest addition to my smartphone is the Topo Maps+ app! By far the my favorite app while hiking. It works on airplane mode and tracks your hike while you go. If you download any maps before you go, they are there waiting for you. Also, you can print any map out at home if you choose to. Go to: https://itunes.apple.com/app/apple-store/id672246353?pt=2135919&ct=BlogReviews&mt=8

I think the transition to backpacking from day adventures doesn't require buying too much at first. If you are going to give it a shot, then don't worry about buying all the top gear. My most expensive set-up in my gear is my sleeping bag, pad, tent, and the backpack itself. Try to borrow these key items from a veteran overnighter; most of which have plenty of older stuff to lend out.

You will begin to add things to your list that you may need or scratch items off the list that are not being used often enough. Also, if you are just going out for one night, your list may vary based on weather. It is important to know that the weather station is not that good at their predictions!

Another reason for a varied list could be numbers in your group. For example, you may want to bring only one stove, and might possibly double up in a tent. Safety items can be spread out amongst those in a group as well.

Here is a great link to another write up on this topic http://www.pmags.com/backpacking-a-beginners-primer