diff --git a/guestbook.go b/guestbook.go
index f237cd2..64db2c6 100644
--- a/guestbook.go
+++ b/guestbook.go
@@ -5,14 +5,27 @@ import (
 	"time"
 )
 
-const DateFormat = "2006-01-02 15:04:05"
+const (
+	// DateFormat is a format of the date and time used in a guestbook.
+	DateFormat = "2006-01-02 15:04:05"
 
+
+// Reply holds a reply for a guestbook's entry.
 type Reply struct {
-	ID      int64     `json:"-"`
+	// ID of a guestbook's entry is used when update of a reply is performed.
+	//
+	// Not present in JSON because a reply comes within its corresponding entry.
+	ID int64 `json:"-"`
+
+	// Created holds a date and time when a reply was created.
 	Created time.Time `json:"created,omitempty"`
-	Message string    `json:"message"`
+
+	// Message holds a reply's message.
+	Message string `json:"message"`
 }
 
+// NewReply creates a new reply and, before that, verifies that a passed message
+// is not empty.
 func NewReply(entryID int64, message string) (*Reply, error) {
 	if message == "" {
 		return nil, errors.New("empty message field")
@@ -24,16 +37,32 @@ func NewReply(entryID int64, message string) (*Reply, error) {
 		Message: message}, nil
 }
 
+// Entry holds a guestbook entry.
 type Entry struct {
-	ID          int64     `json:"entry_id"`
-	Created     time.Time `json:"created"`
-	Name        string    `json:"name"`
-	Website     string    `json:"website,omitempty"`
-	HideWebsite bool      `json:"hide_website,omitempty"`
-	Message     string    `json:"message"`
-	Reply       *Reply    `json:"reply,omitempty"`
+	// ID of an entry.
+	ID int64 `json:"entry_id"`
+
+	// Created holds the date and time when an entry was created.
+	Created time.Time `json:"created"`
+
+	// Name holds a nick-/name of a guest who posted an entry.
+	Name string `json:"name"`
+
+	// Website of a guest. Can be empty.
+	Website string `json:"website,omitempty"`
+
+	// HideWebsite tells wether to hide or show guest's website.
+	HideWebsite bool `json:"hide_website,omitempty"`
+
+	// Message that a guest wrote.
+	Message string `json:"message"`
+
+	// Reply holds a reply for this entry.
+	Reply *Reply `json:"reply,omitempty"`
 }
 
+// NewEntry creates a new Entry and, before that, verifies if the name and message
+// aren't empty and the website field do not exceed the limit.
 func NewEntry(name, message, website string, hideWebsite bool) (*Entry, error) {
 	if name == "" || message == "" {
 		return nil, errors.New("name and message field are required")
@@ -47,14 +76,24 @@ func NewEntry(name, message, website string, hideWebsite bool) (*Entry, error) {
 		Message:     message}, nil
 }
 
+// Guestbook is an interface that should be implemented by a DB.
 type Guestbook interface {
+	// Entries returns a slice of guestbook entries.
 	Entries(page, pageSize int64) ([]*Entry, error)
+	// Count returns how much entries are in a DB.
 	Count() (int64, error)
+	// NewEntry inserts a given Entry into a DB.
 	NewEntry(entry *Entry) error
+	// EditEntry modifies the fields of an existing entry.
 	EditEntry(entry *Entry) error
+	// DeleteEntry removes an Entry with a given ID.
 	DeleteEntry(entryID int64) error
+	// NewReply inserts a given Reply into a DB.
 	NewReply(reply *Reply) error
+	// EditReply modifies a message field of a given Reply.
 	EditReply(reply *Reply) error
+	// DeleteReply removes a Reply with a given ID of an Entry.
 	DeleteReply(entryID int64) error
+	// Close closes a DB.
 	Close() error
 }