A cisco style filter is comprised of one or more of the following rules. Anything in italics is defined following the command syntax. Anything in bold face is a literal.

interface interface-name
All following access-group rules will be applied only to interface-name (until a new interface command is issued)
ip access-group number
ip access-group number in
ip access-group number out
Establish the direction and assign the current interface (if any) to the rule set. The direction simply determines if the interface must match the input or output interface.
access-list number action srcaddr mask
Add a rule the ruleset indicated for packets matching the given source address with the applied mask.
ale
ale ESTABLISHED
ale op dstport
ale op dstport ESTABLISHED
ale SRCPORT op srcport
ale SRCPORT op srcport ESTABLISHED
ale op dstport SRCPORT op srcport
ale op dstport SRCPORT op srcport ESTABLISHED
An extended rule that allows specification of the destination address as well as the destination and source ports. The addition of established means the (TCP) packet has either the ACK or RST bit set. This will *not* match SYN packets!

ale
access-list-extended number action proto srcaddr mask dstaddr mask
proto
IP, ICMP, IGMP, GGP, IPIP, TCP, EGP, PUP, UDP, IDP, TP, EON, ENCAP
action
permit, permit[number], deny, deny[number]

if number is specified then that many bytes will be sent to the IPFW logging socket. A value of 0 implies the whole packet should be sent.

op
lt, gt, eq, neq
interface-name
the name of a BSD/OS interface, e.g., exp0
number
a numeric value
srcaddr
the IP address which must match the packet's source address
dstaddr
the IP address which must match the packet's destination address
mask
an inverse mask (e.g., 0.0.0.255 is really 255.255.255.0) to apply to the source or destination address of the packet before comparing to srcaddr or dstaddr
dstport
the port number which must match the destination port of the packet
srcport
the port number which must match the source port of the packet

The ipfwcisco does not attempt to support all features found in the cisco filter language. One notable feature that is missing is the term any. I believe that 0.0.0.0 255.255.255.255 is the same thing.