Users are required to stake resources to gain the right to manage EOS.
RAM refers to the storage status. RAM would be spent during EOS name bidding and SR voting, etc. RAM transaction actually is staking EOS to system account instead of a real transaction between seller and buyer. Buy RAM=Stake EOS; Sell RAM=Reduce Stake EOS
CPU&NET is two kinds of bandwidth of EOS account to restrict the blockchain resources each EOS account can use. All the transaction of EOS blockchain requires CPU&NET resources. Different from RAM, EOS stake for bandwidth would be refunded back to the available balance in 72 hours after staking.
If users want to vote for EOS SRs, CPU&NET resources are also required to get the equal amount of votes.