Ada held on the Cardano network represents a stake in the network, with the size of the stake proportional to the amount of ada held. If you hold 10% of all available ada, you have a 10% stake in the network. 

 The ability to delegate that stake is fundamental to how Cardano works. There are two ways an ada holder can earn rewards, and thus be incentivized to keep the network running: by delegating their stake to a stake pool run by someone else, or running their own stake pool. The amount of stake delegated to a given stake pool is the primary way the Ouroboros protocol chooses who should add the next block to the blockchain, and get a monetary reward for doing so. The more stake is delegated to a stake pool (up to a certain point), the more likely it is to make the next block – and then the rewards are shared between everyone who delegated their stake to that stake pool.   

 Stake pools are nodes delegated to by ada holders that do not have the knowledge, resources, or desire to run a node. Conversely, stake pool operators are those, like me, with the expertise and resources to reliably operate a node 24/7. Watch this delegation summary video for more information. 

Participating in the Cardano protocol incurs far less computational costs than participating in Bitcoin. Nevertheless, having slot leaders online when it is their turn to create a block or to participate in the election process is important for both the security and efficiency of the protocol. Thus it is our goal to keep our server on-line 24/7/365

While Ouroboros is cheaper to run than a proof of work protocol, running Ouroboros still incurs some costs: electricity, network connectivity, equipment depreciation.etc.

Therefore, stake pools are rewarded for running the protocol in the form of incentives that come from the transaction fees and from inflation of the circulating supply of ada.

 rADAr charges the minimum fixed cost (340A per epoch) and a low margin of 2.9%

  Daedalus is a full node, meaning you must download the entire blockchain to use it. It's developed by IOHK, the blockchain research and development company behind Cardano. It is very safe. Only you control your keyes. Here is a link to the currecnt Daedalus Wallet. As always be sure to run either or both of the Security Checks listed with the wallet. (Trust but verify!)

 With Daedalus you can create paper wallets. In my opinion this is probably best option for security until Ledger Wallet becomes available. Print the pdf Verify the key, and place a copy of the PDF in your Safe Deposit box.  

Very soon there will be Ledger and Trezor Shelley support for both Daedalus and Yoroi

 Yoroi is a light wallet for Cardano. It’s simple, fast and secure. You can stake right from Yoroi. As a Chrome plugin, Yoroi looks to be a day to day wallet for a Cardano user. 

Innovative and  Fast! Yoroi is a light wallet for Cardano. There is no need to download the blockchain when you open the wallet. So you are ready to send and receive transactions right away. 

 Secure with High-quality code, thoroughly tested, security audited and more to make sure that Yoroi works flawlessly. Private keys are encrypted and never shared with Yoroi servers or third party providers. 

You will soon be able to recreate paper wallets and stake from Yoroi. You can also use Yoroi with Ledger and Trezor