I have already discussed previously the terminology involved in the hierarchy of abstractions when talking about Exadata disks. When learning new terminology I always think some form of diagram can make things much, much clearer. Hopefully the following diagrams will be of use to you too.
First up, somewhat simplistically, how ordinary storage cell drives are carved up with griddisks:
So we see here how multiple griddisks are created on top of 1 celldisk, though they have been labelled with the name of the ASM Diskgroup that they belong to. The thing to take away though, is the multiple griddisks presented to ASM from the 1 celldisk.
A bit more interesting is how a storage cell system disk looks like:
So we see that there is no griddisk associated with the SYSTEM diskgroup (yes, newer storage cell versions have this called DBFS_DG). However we note that there are a multitude of additional partitons to hold the storage cell software, and (assuming an upgrade has been done) one previous version of the cell software.
It should be noted that there is still 1 celldisk created here on this system disk, and the two griddisks are still as normal carved out from this.
Finally a look at how the full hierarchy stacks up:
So here we see how all the terminology stacks up, with the actual physical disk at the bottom of the hierarchy.
The top pieces of the hierarchy are of course the ASM diskgroups upon which you can create your database datafiles.