IOPs are an attempt to standardize comparison of disk speeds across different environments. When you turn on a computer, everything must be read from disk, but thereafter things are kept in memory. However applications typically read and write to disk frequently. When you move to enterprise class applications, especially relational databases, a lot of disk I/O is happening so performance of disk resources is crucial.
For a basic single SATA drive that you might have in server or laptop, you can typically get 30-40 IOPs from it. These numbers vary if you are talking about random versus sequential reads or writes. Picture the needle on a vinyl record. It moves quicker around the center, and slower around the outside. That’s what’s happening the the magnetic needle inside your harddrive too.
In Amazon EC2 environment, there is a lot of variability in performance from EBS. You can stripe across four separate EBS volumes which will be on four different locations on the underlying RAID array and you’ll get a big boost in disk I/O. Also disk performance will vary from an m1.small, m1.large and m1.xlarge instance type, with the latter getting the lions share of network bandwidth, so better disk I/O performance. But in the end your best EBS performance will be in the range of 500-1000 IOPs. That’s not huge by physical hardware standards, so an extremely disk intensive application will probably not perform well in the Amazon cloud.
Still the economic pressures and infrastructure and business flexibility continue to push cloud computing adoption, so expect the trend to continue.