ovftool …

OVF is an open-standard for VM distribution. OVF has a number of important benefits, like encapsulating the entire VM in a single file, storing metadata, support for multi-VM configurations, disk compression, and platform independence. VC 2.5 also allows the VI client to import an OVF image from a URL and download it and install it automatically over the
internet.

In order to create an OVF image, you can use the OVF tool that VMware provides to convert an existing VM into an OVF image.

You can get more information about OVF from:
http://www.vmware.com/appliances/learn/ovf.html

docs here:

Click to access ovf_tool.pdf

You can download the OVF tool from:
http://www.vmware.com/download/eula/ovf_eula.html

In terms of building and packaging the VA, a good starting point is the VA build page: http://www.vmware.com/appliances/build/how.html. I suggest you also take a look at the VA best practices doc (http://www.vmware.com/files/pdf/vam_best_practices_building.pdf), which covers issues about how to package and distribute virtual appliances. A few key suggestions from the paper including using virtual SCSI (and not IDE) drives, disconnecting virtual CDROMs, floppies, etc, and installing VMware tools. If you follow the best practice guidelines, you can request to certify the virtual appliance as well. Those guidelines are at http://www.vmware.com/appliances/build/list.html. I suggest you read and follow the best practices paper whether you plan to certify or not, as it will allow maximum portability for your virtual appliance.