Optimistic doesn’t try to lock the data until it
goes to do the update.  Pessimistic locks the data
as soon as it is changed and doesn’t release the
lock until the data is updated on disk.

Optimistic does not monopolize the data and can
make a multiuser app work smoother, but pessimistic
will guarantee that the user will never see the
“Another user has updated that record already”
message.

You mention “client/server” which leads me to
believe you are using a back-end that is not
a VFP database?  If so, there are some
different considerations regarding impact on
the performance of database access.  I have
yet to do a client/server app in VFP, so I
will yield the floor to someone who has…