The ESM file format stands for Elder Scrolls Master file. It has was originally used by Bethesda for Morrowind, and was first used in Fallout 3 in the Fallout series. It is a binary file which contains core game logic and other resources and is depended upon by other file types such as the BSA file format.
Overview of record types found in Fallout3.esm
which control many internal functions within the game, such as run speed, jump height, how much skills affect damage, how easy it is to sneak, how condition affects weapons, and many other neat things.
TXST -- Texture Settings (?). These seem to be related to what sorts of textures are being applied to Object. Props used in animations (fork and knife, bottles, cups, cigaretts).
WATR --Water. Controls flags related to water by type, in Fallout 3, this would be color, cloudyness, radiation, and similar (unconfirmed).
EFSH --Effect Shader. Controls appearance of shaders.
EXPL --Explosion. Controls appearance, size, and force of explosions (unconfirmed).
DEBR --Debris. Defines debris created when blowing off limbs, ect (unconfirmed).
IMGS --Image Space. Unknown
IMAD --Unknown. Related to certain visual effects.
FLST --Unknown. Fallout Lists? Seems to be related to repair or durability effects, but applies to other things.
PERK --Perk. Defines Perks. Much of what is related to a perk is controlled elsewhere, or is hard coded. (unconfirmed)
BPTD --Body Part Data. Unknown, may be VATS related.
ADDN --Addon (?). Seems related to temporary visuals caused by an item like; cigarette smoke, gun sparks, blood spatter.
AVIF --Actor Value Something Something (?). Seems to relate to Actor Values such as skills, S.P.E.C.I.A.L., and other things which impact actor ability. (unconfirmed)
RADS --Unknown. Radiation related (unconfirmed).
CAMS --Cameras. Used in VATS. (unconfirmed)
CPTH --Unknown. Seems related to CAMS, used in VATS. (unconfirmed)
VTYP --Voice Type. Used by dialogues/NPC (?) to create variations in voices.
IPCT --Impact. Determines effects on impact (unconfirmed).
IPDS --Impact Data Set. Controls other things related to impacts (unconfirmed).
ARMA --Arm Addon (?). Defines non-functional items (gloves, backpacks, hoses) to an actor.
ECZN --Unknown. Seems to be related to conditional respawns.
MESG --Message. Defines various message scrings.
RGDL --Ragdoll. Related to ragdoll attributes (unconfirmed).
DOBJ --Default Object Manager. Unknown.
LGTM --Lighting Template (?). Related to interiors, may only be used in the creation of new cells, rather than altering existing ones. (unconfirmed)
MISC --Music. Music Settings.
! ! This list was compilled going in order as the entries appear in TESsnip. Further organization and refinement may be needed. ! !
Many of the things listed above probably will not be made clear, or be made viable for modding until an official CS is released. However, attempts have been made to help facilitate people who would be willing to research further. Things listed here may not be 100% accurate, but are as accurate as can be determined by the author at this time. Regardless of completeness or correctness, this is better, and more useful than an empty page. Good Luck.