Isolinux files:
isolinux.bin - boot loader for no-emulation boot
memdisk - this file is used to load floppy and HDD images
isolinux.cfg - configuration file
*.txt / *.msg - text files with some commands. This files are used for making boot messages - splash screens
*.lss - Isolinux graphic splash screen

isolinux.bin
This file is loader himself. If you are making bootable CD that uses Isolinux for booting, then point your CD-burning software to this file when it ask for boot sector.

Memdisk
This file is loader for floppy and HDD images. Memdisk is called by isolinux.bin according to options specified in isolinux.cfg.
For a floppy image, the size of the image should be exactly one of the following:

1,228,800 bytes - For a 1200K floppy image
1,474,560 bytes - For a 1440K floppy image
2,949,120 bytes - For a 2880K floppy image

Any other size is assumed to be a hard disk image.
See isolinux.cfg for more details on using memdisk.

isolinux.cfg
This is configuration file for isolinux. It is a plain text file and can be edited in any text editor.
Options in isolinux.cfg  DON'T have a particular order. Isolinux.cfg can be in DOS or Linux text format.

Items/commands in isolinux.cfg:

#  - comment, use this at the begining of line to insert comments

DEFAULT {label name or kernel options} - specifies what will be booted after boot timeout or if user just hits ENTER without specifiing boot options.
example:
DEFAULT myDOS - this will work just like you have typed myDOS at boot prompt, that means - it will boot anything that is specified in label myDOS (see label item)
DEFAULT kernel bootsec.bin - this will boot bootsec.bin (for example WindowsXP installation CD boot sector)

LABEL {label name} - This is a section that explains boot options for {label name}.
example 1:
LABEL myDOS
    kernel memdisk
    append initrd=DOS.IMG
This means: if user types myDOS at boot prompt then use memdisk (as kernel) to load DOS.IMG (floppy image file).
example 2:
LABEL oldbootcd
    kernel bootsec.bin
This means: if user types oldbootcd at boot prompt then load CD boot sector as kernel (this can be used if you integrate more bootable CDs to your multiboot CD or to load any other no-emulation boot sector). With this method you can load BCDW, BScriptor or CDShell from Isolinux ( see Links section ).
example 3:
LABEL linux
    kernel vmlinuz
    append max_loop=255 initrd=initrd.gz init=linuxrc livecd_subdir=/ ...
This means: if user types linux at boot prompt then load linux kernel and pass parameters from append section to linux kernel (used for live linux CDs).
example 4:
LABEL hdd
    localboot 0x80
This means: if user types hdd at boot prompt then boot from first partition of first HDD.
Valid parameters for localboot are:
0x80 - first partition of first HDD (C: in DOS)
0x81 - 2nd partition of first HDD (D: in DOS)
0x00 - first floppy drive (A: in DOS)
....etc
-1 - tell BIOS to boot from next device listed in BIOS
example 5 :
LABEL somethig
    kernel memdisk
    append myFolder/myDisk.img
This is example how you can use folders. In this example isolinux will search for myDisk.img in subfolder of folder that contains isolinux.cfg (in most cases this is /Isolinux folder). If isolinux.cfg is in folder /Isolinux, then myDisk.img should be in /Isolinux/myFolder.
example 6:
LABEL something
    kernel /vmlinuz
    append max_loop=255 initrd=/initrd.gz init=linuxrc livecd_subdir=/ ...
This one will search for vmlinuz in a parent folder of folder that contains isolinux.cfg. It will also search for initrd.gz in parent folder ( "/" before vmlinuz and before initrd.gz). If isolinux.cfg folder is /Isolinux then files will be searched in root folder of the CD.
example 7:
LABEL something
    kernel /livelin/vmlinuz
    append max_loop=255 initrd=/livelin/initrd.gz init=linuxrc livecd_subdir=/ ...
This one will search for vmlinuz and initrd.gz in following way: go to the parent folder of folder containing isolinux.cfg  (in most cases this if /Isolinux folder), now go  to the subfolder livelin and find files there. Kernel file and initrd file don't need to be in the same folder.

TIMEOUT x - set timeout in 1/10 sec. If user don't type anything at boot prompt in time set by TIMEOUT, then DEFAULT or ONTIMEOUT will be executed ( see ONTIMEOUT for details).

ONTIMEOUT {label name or kernel options} - same as DEFAULT. If ONTIMEOUT option is set, then DEFAULT is used just if user hits ENTER without options at boot prompt.

DISPLAY filename - display text (splash screen) specified with filename before boot prompt (at isolinux start). Splash screen is text file that can contain some commands. Extension of splash screen files can be ANY (.txt .msg .xyz...) except of:  .0 .bin .bs .bss .c32 .cbt .com .img

F1  filename - display text (same kind of text files as for DISPLAY) if user press F1. Same for F2 F3 ... F9 F0.

SAY message - print the message on the screen

PROMPT x - If x is 0, display the boot prompt only if the Shift or Alt key is pressed, or Caps Lock or Scroll lock is set (this is the default). If x is 1, always display the boot prompt. My advice: - use PROMPT 1.

Notes:
-command names are case insensitive, but label names are case sensitive.
-isolinux.bin will search for isolinux.cfg file in following folders on CD: / (root of CD), /boot, /isolinux

Example of isolinux.cfg:

# default to boot from hard drive
default 0
# let the user know the basic options
display WELCOME.TXT
prompt 1
# various options are available for booting

# hd
label 0
localboot 0x80

# floppy
label a
localboot 0x00

# cancel and try next boot device
label q
localboot -1

# MY BOOT
label mydos
kernel memdisk
append initrd=BOBBY71p.IMG

# MY BOOT2
label mydos2
kernel memdisk
append initrd=BOBBY71s.IMG

# part image
label 11
kernel partimg/vmlinuz
append initrd=partimg/initrd.img load_ramdisk=1 prompt_ramdisk=0 rw root=/dev/ram
 
# movix vesa
label VE
kernel kernel/vmlinuz
append initrd=initrd.gz load_ramdisk=1 prompt_ramdisk=0 ramdisk_size=62000 rw root=/dev/ram0 LABEL=vesa video=vesa:off

timeout 200
F1 DOS.TXT
F2 MOVIX2.txt
F3 UTIL.TXT
F4 LINUX.TXT
F5 MX2HELP.TXT
F6 TRBLST.TXT
F7 MPHELP.TXT
F8 PARTIMG.TXT
F9 WELCOME.TXT
F0 WELCOME.TXT