They're just structs with a bunch of information. For example, Elf32_Ehdr is defined as such:
typedef struct {
unsigned char e_ident[EI_NIDENT]; /* ident bytes */
Elf32_Half e_type; /* file type */
Elf32_Half e_machine; /* target machine */
Elf32_Word e_version; /* file version */
Elf32_Addr e_entry; /* start address */
Elf32_Off e_phoff; /* phdr file offset */
Elf32_Off e_shoff; /* shdr file offset */
Elf32_Word e_flags; /* file flags */
Elf32_Half e_ehsize; /* sizeof ehdr */
Elf32_Half e_phentsize; /* sizeof phdr */
Elf32_Half e_phnum; /* number phdrs */
Elf32_Half e_shentsize; /* sizeof shdr */
Elf32_Half e_shnum; /* number shdrs */
Elf32_Half e_shstrndx; /* shdr string index */
} Elf32_Ehdr;
You can find all of them defined here:
http://www.opensource.apple.com/source/dtrace/dtrace-78/sys/elf.h