/etc/skelディレクトリには、ユーザが新たに作成された際に、そのユーザのホームディレクトリにコピーされるディレクトリやファイルが配置されています。

user@debian:~$ ls -a /etc/skel
.  ..  .bash_logout  .bashrc  .profile

/etc/default/useraddファイルは、新たに作成されるユーザのデフォルトの設定を定義するファイルです。
そのファイルで定義されている変数の一部の意味は以下の通りです。

SHELL: ログインシェル
GROUP: グループID(GID)
HOME: ホームディレクトリ

user@debian:~$ cat /etc/default/useradd
# Default values for useradd(8)
#
# The SHELL variable specifies the default login shell on your
# system.
# Similar to DSHELL in adduser. However, we use "sh" here because
# useradd is a low level utility and should be as general
# as possible
SHELL=/bin/sh
#
# The default group for users
# 100=users on Debian systems
# Same as USERS_GID in adduser
# This argument is used when the -n flag is specified.
# The default behavior (when -n and -g are not specified) is to create a
# primary user group with the same name as the user being added to the
# system.
# GROUP=100
#
# The default home directory. Same as DHOME for adduser
# HOME=/home
#
# The number of days after a password expires until the account 
# is permanently disabled
# INACTIVE=-1
#
# The default expire date
# EXPIRE=
#
# The SKEL variable specifies the directory containing "skeletal" user
# files; in other words, files such as a sample .profile that will be
# copied to the new user's home directory when it is created.
# SKEL=/etc/skel
#
# Defines whether the mail spool should be created while
# creating the account
# CREATE_MAIL_SPOOL=yes