VMware products have different kinds of disks. Some are compatible with ESX some are not. To make them compatible we have to use tools like converter OR vmkfstools
1) Zeroedthick (default) – This is the default type of disk format. All the space is allocated at creation time. Any data remaining on the physical device is not erased during creation, but is zeroed out on demand at a later time on first write from the virtual machine. These types of disks are less secure because their disk blocks are not being cleared of previous data. The performance may not be great at first write because the disk needs to be zeroed before the data can be written.
2) Eagerzeroedthick – Space required for the virtual disk is allocated at creation time.Since the data remaining on the physical device is zeroed out during creation, it takes longer time to create this disk. These disks are the most secure and, because the block has already been cleared of previous data, offer slightly better performance on the first write of data to a disk block.
3) Thick disks – Space required for the virtual disk is allocated during creation. This type of formatting does not zero out any old data that might be present on this allocated space.
4) Thin Disk – Space required for the virtual disk is not allocated during creation, but is supplied and zeroed out, on demand at a later time.
5) RDM – Virtual compatibility mode for raw disk mapping
6) RDMP – Physical compatibility mode (pass-through) for raw disk mapping
7) RAW – Raw device
8) 2bgsparse disk – 2bgsparse divides a disk into multiple disks with a maximum size of 2 GB per disk. You can use disks in this format with other VMware products like Workstation, Server. You cannot use this type of disks in ESX server. You will have to import them with the help of converter OR vmkfstools
9) monosparse – A monolithic sparse disk. You can use disks in this format with other VMware products.
10) monoflat – A monolithic flat disk. You can use disks in this format with other VMware products.
The above information is from VMware's kb article 1022242