Rails ActiveRecord Transaction vs Class Transaction

I have a doubt about Rails Transaction type. What the diffence between them and whem to use each one ?

Active Record Transactions

ActiveRecord::Base.transaction do
    david.withdrawal(100)
    mary.deposit(100)
end

Class Transactions

Account.transaction do
    balance.save!
    account.save!
end

Instance Transactions

account.transaction do
    balance.save!
    account.save!
end

Thank you very much in advance!