Modules
Here is a list of all modules:

The db-lib API | Functions callable by `db-lib` client programs
Primary functions | Core functions needed by most db-lib programs.
Remote Procedure functions | Functions used with stored procedures. Especially useful for OUTPUT parameters, because modern Microsoft servers do not return output parameter data to the client unless the procedure was invoked with dbrpcsend().
Bulk copy functions | Functions to bulk-copy (a/k/a bcp) data to/from the database.
Internal bcp functions | Static functions internal to the bcp library.
Money functions | Functions to manipulate the MONEY datatype.
Datetime functions | Functions to manipulate DBDATETIME structures. Defined by Sybase only. These are not implemented:
Internals | Functions called within `db-lib` for self-help. These functions are of interest only to people hacking on the FreeTDS db-lib implementation.

Unimplemented

Functions thus far not implemented in the FreeTDS db-lib implementation.
While some of these are simply awaiting someone with time and skill (and inclination) it might be noted here that the old browse functions (e.g. dbcolbrowse()) are on the never-to-do list.
They were defined by Sybase and were superseded long ago, although they're still present in Microsoft's implementation.
They were never popular and today better alternatives are available.
For completeness, they are:

ODBC API | Functions callable by `ODBC` client programs
ODBC utility | Functions called within `ODBC` driver
LibTDS API | Callable functions in `libtds`
Authentication | Functions for handling authentication
Configuration | Handle reading of configuration
Conversion | Conversions between datatypes. Supports, for example, dbconvert().
Charset conversion | Convert between different charsets

Memory allocation

Allocate or free resources. Allocation can fail only on out of memory. In such case they return NULL and leave the state as before call. Mainly function names are in the form tds_alloc_XX or tds_free_XXX. tds_alloc_XXX functions allocate structures and return pointer to allocated data while tds_free_XXX take structure pointers and free them. Some functions require additional parameters to initialize structure correctly. The main exception are structures that use reference counting. These structures have tds_alloc_XXX functions but instead of tds_free_XXX use tds_release_XXX

Query | Function to handle query
Network functions | Functions for reading or writing from network
Results processing | Handle tokens in packets. Many PDU (packets data unit) contain tokens. (like result description, rows, data, errors and many other)
Dynamic string functions | Handle dynamic string. In this string are always valid (you don't have NULL pointer, only empty strings)