Here are the data structures with brief descriptions:

Calpm_backup_t	Local package or package file backup entry
Calpm_conflict_t	A conflict that has occurred between two packages
Calpm_depend_t	The basic dependency type
Calpm_depmissing_t	Missing dependency
Calpm_download_event_completed_t	Context struct for when a download completes
Calpm_download_event_init_t	Context struct for when a download starts
Calpm_download_event_progress_t	Context struct for when a download progresses
Calpm_download_event_retry_t	Context struct for when a download retries
Calpm_event_any_t	An event that may represent any event
Calpm_event_database_missing_t	A database is missing
Calpm_event_hook_run_t	A pre/post transaction hook was ran
Calpm_event_hook_t	Pre/post transaction hooks are to be ran
Calpm_event_optdep_removal_t	An optional dependency was removed
Calpm_event_package_operation_t	A package operation event occurred
Calpm_event_pacnew_created_t	A pacnew file was created
Calpm_event_pacsave_created_t	A pacsave file was created
Calpm_event_pkg_retrieve_t	Packages downloading about to start
Calpm_event_pkgdownload_t	A package was downloaded
Calpm_event_scriptlet_info_t	A scriptlet was ran
Calpm_event_t	Events
Calpm_file_t	File in a package
Calpm_fileconflict_t	File conflict
Calpm_filelist_t	Package filelist container
Calpm_graph_t
Calpm_group_t	Package group
Calpm_list_t	A doubly linked list
Calpm_mountpoint_t
Calpm_pgpkey_t	A PGP key
Calpm_pkg_xdata_t	The extended data type used to store non-standard package data fields
Calpm_question_any_t	A question that can represent any other question
Calpm_question_conflict_t	Should a conflicting package be removed?
Calpm_question_corrupted_t	Should a corrupted package be deleted?
Calpm_question_import_key_t	Should a key be imported?
Calpm_question_install_ignorepkg_t	Should target in ignorepkg be installed anyway?
Calpm_question_remove_pkgs_t	Should unresolvable targets be removed from the transaction?
Calpm_question_replace_t	Should a package be replaced?
Calpm_question_select_provider_t	Provider selection
Calpm_question_t	Questions
Calpm_siglist_t	Signature list
Calpm_sigresult_t	Signature result
Calpm_trans_t
Carchive_read_buffer	Used as a buffer/state holder for _alpm_archive_fgets()
Cdb_operations
Cdload_payload
Ckeyinfo_t
Cpackage_changelog
Cpkg_operations	Package operations struct
Cserver_error_count