diff --git a/docs/mypy.md b/docs/mypy.md
index e4b8b8f596..3d73148c73 100644
--- a/docs/mypy.md
+++ b/docs/mypy.md
@@ -158,3 +158,8 @@ to annotate strings in Zulip's code.  We follow the style of doing
 of doing `import six` and using `six.text_type` for annotation, because
 `text_type` is used so extensively for type annotations that we don't
 need to be that verbose.
+
+Sometimes you'll find that you have to convert strings from one type to
+another.  `zerver/lib/str_utils.py` has utility functions to help with that.
+It also has documentation (in docstrings) which explains the right way
+to use them.
diff --git a/zerver/lib/str_utils.py b/zerver/lib/str_utils.py
index dc465d585a..d62434877b 100644
--- a/zerver/lib/str_utils.py
+++ b/zerver/lib/str_utils.py
@@ -1,3 +1,35 @@
+"""
+String Utilities:
+
+This module helps in converting strings from one type to another.
+
+Currently we have strings of 3 semantic types:
+
+1.  text strings: These strings are used to represent all textual data,
+    like people's names, stream names, content of messages, etc.
+    These strings can contain non-ASCII characters, so its type should be
+    six.text_type (which is `str` in python 3 and `unicode` in python 2).
+
+2.  binary strings: These strings are used to represent binary data.
+    This should be of type six.binary_type (which is `bytes` in python 3
+    and `str` in python 2).
+
+3.  native strings: These strings are for internal use only.  Strings of
+    this type are not meant to be stored in database, displayed to end
+    users, etc.  Things like exception names, parameter names, attribute
+    names, etc should be native strings.  These strings should only
+    contain ASCII characters and they should have type `str`.
+
+There are 3 utility functions provided for converting strings from one type
+to another - force_text, force_bytes, force_str
+
+Interconversion between text strings and binary strings can be done by
+using encode and decode appropriately or by using the utility functions
+force_text and force_bytes.
+
+It is recommended to use the utility functions for other string conversions.
+"""
+
 import six
 from six import text_type, binary_type
 from typing import Any, Mapping, Union, TypeVar
@@ -7,6 +39,7 @@ NonBinaryStr = TypeVar('NonBinaryStr', str, text_type)
 
 def force_text(s):
     # type: (Union[text_type, binary_type]) -> text_type
+    """converts a string to a text string"""
     if isinstance(s, text_type):
         return s
     elif isinstance(s, binary_type):
@@ -16,6 +49,7 @@ def force_text(s):
 
 def force_bytes(s):
     # type: (Union[text_type, binary_type]) -> binary_type
+    """converts a string to binary string"""
     if isinstance(s, binary_type):
         return s
     elif isinstance(s, text_type):
@@ -25,6 +59,7 @@ def force_bytes(s):
 
 def force_str(s):
     # type: (Union[text_type, binary_type]) -> str
+    """converts a string to a native string"""
     if isinstance(s, str):
         return s
     elif isinstance(s, text_type):