iface eth0 inet static: Defines a static IP address for eth0

iface eth0 inet manual :To create a network interface without an IP address at all. Usually used by interfaces that are bridge or aggregation members, or have a VLAN device configured on them

For more information read:

• https://wiki.debian.org/NetworkConfiguration

Take a look on the file /usr/share/doc/ifupdown/examples/network-interfaces.gz, you can know more about manual and some cases to use:

# Set up an interface to read all the traffic on the network. This 
# configuration can be useful to setup Network Intrusion Detection
# sensors in 'stealth'-type configuration. This prevents the NIDS
# system to be a direct target in a hostile network since they have
# no IP address on the network. Notice, however, that there have been
# known bugs over time in sensors part of NIDS (for example see 
# DSA-297 related to Snort) and remote buffer overflows might even be
# triggered by network packet processing.
# 
# auto eth0
# iface eth0 inet manual
#   up ifconfig $IFACE 0.0.0.0 up
#       up ip link set $IFACE promisc on
#       down ip link set $IFACE promisc off
#       down ifconfig $IFACE down

# Set up an interface which will not be allocated an IP address by
# ifupdown but will be configured through external programs. This
# can be useful to setup interfaces configured through other programs,
# like, for example, PPPOE scripts.
#
# auto eth0
# iface eth0 inet manual
#       up ifconfig $IFACE 0.0.0.0 up
#       up /usr/local/bin/myconfigscript
#       down ifconfig $IFACE down