Oyster card

An oyster card is like an electronic debit card for the London transit system. You load money into the card, wave it over at the turnstiles and the fare gets deducted from the cash loaded into the card. When you have taken enough trips to qualify for an all day-pass that money will be credited back to you so in the end you will always get the best deal. When your trip to London is finished you can get a refund for the unused funds on the card. Buying individual tickets is way more expensive to encourage people to get the Oyster.
Another alternative is you use a smartphone with Apple Pay or the Android equivalent at the turnstiles instead of a separate Oyster Card, you will get the same pricing.
http://tinyurl.com/8drb8zw

Official site with same info
https://tfl.gov.uk/fares-and-payments/oyster/what-is-oyster?intcmp=1685

We loved using the Oyster cards when we were in London 3 years ago. Its so easy to just swipe and go! Plus if you need more, you can just add more money. Super easy. Enjoy your trip.

https://www.tripadvisor.co.uk/Travel-g186338-c133479/London:United-Kingdom:Paper.And.Oyster.Travelcards.2.4.1.Offers.html

Each card will cost you 5 GBP but that will be refunded when you return the cards.

With some of the lines at larger tube stations to get tickets, you will save time, and money, with the Oyster card. And remember, if you encounter any stations without exit gates, look for the ticket/Oyster reader and wave the card so you don't get charged more than you owe.

the tube is much more expensive if you pay cash

buses don't take cash

so will you need an Oyster card or another means of electronic payment

Oyster cards can be purchased at any Tube station where you can buy an individual ticket. Each traveler requires an individual card, they can't be shared. Oyster is definitely a more affordable option over buying individual tickets, and easier too since you only have to buy the Oyster once.

Important to note that if you want to cash out the Oyster when you leave, in order to get a cash refund on the spot you can't have more than £10 left on it and all money put on the card must have been from the same source, that is all cash or all from the same credit card. If you mix payments they will mail you a check (in pounds). You can of course just keep it for your next visit. That's what I have done and my 10 year old Oyster works perfectly every time.

User instructions: The Oyster card lets you into the Tube. To exit, you apply the card again at the exit turnstile. Fares on the Tube are zoned so the appropriate amount for your travel is removed at the end of the trip.

Something that other people haven't mentioned - on the underground you scan your Oyster card at both entry and exit. On buses you only do it when you get on - if you do it when you get off you will be charged double.

Thank you everyone for your help! Lots of good advice and I appreciate your tips! Sounds easy enough and we will definitely be getting them!

Can you buy your Oyster card at Heathrow?

We land LHR have a 5 hour layover. Fly to Dublin then back the next week for 4 days in London. Thinking of things to do during the layover so we can hit the ground running when we return.