In Unix, files and directories beginning with a period usually contain settings for a specific program/a series of programs. Dot files are usually hidden from the user, so you would need to run ls -a to see them.

An example of a dot file is .bash_history (opens new window), which contains the latest executed commands, assuming the user is using Bash.

There are various files that are sourced (opens new window) when you are dropped into the Bash shell. The image below, taken from this site (opens new window), shows the decision process behind choosing which files to source at startup.

(opens new window)

shopt -q login_shell && echo 'login' || echo 'not-login'

Remarks

Login Shell

A login shell is one whose first character of argument zero is a -, or one started with the –login option. The Initialization is more comprehensive than in an normal interactive (sub) shell.

Interactive Shell

An interactive shell is one started without non-option arguments and without the -c option whose standard input and error are both connected to terminals (as determined by isatty(3)), or one started with the -i option. PS1 is set and $- includes i if bash is interactive, allowing a shell script or a startup file to test this state.

non-interactive Shell

A non-interactive Shell is a shell in which the user can not interact with the shell. As en example, a shell running a script is always a non-interactive shell. All the same, the script can still access its tty.

Configuring a login shell

On logging in:

If '/etc/profile' exists, then source it. 
If '~/.bash_profile' exists, then source it, 
else if '~/.bash_login' exists, then source it, 
else if '~/.profile' exists, then source it. 

For non-login interactive shells

On starting up:

If `~/.bashrc' exists, then source it.

For non-interactive shells

On starting up: If the environment variable ENV is non-null, expand the variable and source the file named by the value. If Bash is not started in Posix mode, it looks for BASH_ENV before ENV.