Running gphoto2

You can launch gphoto2 from the command line, just enter

gphoto2

Now because you didn't specify any parameters, gphoto2 will show you a list of available commands.

Usage: gphoto2 [-?valLnPTDR] [-?|--help] [--usage] [--debug] [--debug-logfile=FILENAME] [--quiet]
[--hook-script=FILENAME] [--stdout] [--stdout-size] [--auto-detect] [--show-exif=STRING]
[--show-info=STRING] [--summary] [--manual] [--about] [--storage-info] [--shell]
[-v|--version] [--list-cameras] [--list-ports] [-a|--abilities] [--port=FILENAME]
[--speed=SPEED] [--camera=MODEL] [--usbid=USBIDs] [--config] [--list-config]
[--list-all-config] [--get-config=STRING] [--set-config=STRING] [--set-config-index=STRING]
[--set-config-value=STRING] [--reset] [--keep] [--no-keep] [--wait-event=COUNT]
[--wait-event-and-download=COUNT] [--capture-preview] [-B|--bulb=SECONDS] [-F|--frames=COUNT]
[-I|--interval=SECONDS] [--reset-interval] [--capture-image] [--trigger-capture]
[--capture-image-and-download] [--capture-movie=COUNT or SECONDS] [--capture-sound]
[--capture-tethered=COUNT] [--trigger-capture] [-l|--list-folders] [-L|--list-files]
[-m|--mkdir=DIRNAME] [-r|--rmdir=DIRNAME] [-n|--num-files] [-p|--get-file=RANGE]
[-P|--get-all-files] [-t|--get-thumbnail=RANGE] [-T|--get-all-thumbnails] [--get-metadata=RANGE]
[--get-all-metadata] [--upload-metadata=STRING] [--get-raw-data=RANGE] [--get-all-raw-data]
[--get-audio-data=RANGE] [--get-all-audio-data] [-d|--delete-file=RANGE] [-D|--delete-all-files]
[-u|--upload-file=FILENAME] [--filename=FILENAME_PATTERN] [-f|--folder=FOLDER] [-R|--recurse]
[--no-recurse] [--new] [--force-overwrite] [--skip-existing]

Or you can use the following for a better description of each command:

gphoto2 --help

This will give the following output:

Usage: gphoto2 [OPTION...]
Common options
-?, --help Print complete help message on program usage
--usage Print short message on program usage
--debug Turn on debugging
--debug-logfile=FILENAME Name of file to write debug info to
--quiet Quiet output (default=verbose)
--hook-script=FILENAME Hook script to call after downloads, captures, etc.
Miscellaneous options (unsorted)
--stdout Send file to stdout
--stdout-size Print filesize before data
--auto-detect List auto-detected cameras
--show-exif=STRING Show EXIF information
--show-info=STRING Show info
--summary Show summary
--manual Show camera driver manual
--about About the camera driver manual
--storage-info Show storage information
--shell gPhoto shell
Get information on software and host system (not from the camera)
-v, --version Display version and exit
--list-cameras List supported camera models
--list-ports List supported port devices
-a, --abilities Display camera/driver abilities
Specify the camera to use
--port=FILENAME Specify device port
--speed=SPEED Specify serial transfer speed
--camera=MODEL Specify camera model
--usbid=USBIDs (expert only) Override USB IDs
Camera and software configuration
--config Configure
--list-config List configuration tree
--list-all-config Dump full configuration tree
--get-config=STRING Get configuration value
--set-config=STRING Set configuration value or index in choices
--set-config-index=STRING Set configuration value index in choices
--set-config-value=STRING Set configuration value
--reset Reset device port
Capture an image from or on the camera
--keep Keep images on camera after capturing
--no-keep Remove images from camera after capturing
--wait-event[=COUNT] Wait for event(s) from camera
--wait-event-and-download[=COUNT] Wait for event(s) from the camera and download new images
--capture-preview Capture a quick preview
-B, --bulb=SECONDS Set bulb exposure time in seconds
-F, --frames=COUNT Set number of frames to capture (default=infinite)
-I, --interval=SECONDS Set capture interval in seconds
--reset-interval Reset capture interval on signal (default=no)
--capture-image Capture an image
--trigger-capture Trigger capture of an image
--capture-image-and-download Capture an image and download it
--capture-movie[=COUNT or SECONDS] Capture a movie
--capture-sound Capture an audio clip
--capture-tethered[=COUNT] Wait for shutter release on the camera and download
--trigger-capture Trigger image capture
Downloading, uploading and manipulating files
-l, --list-folders List folders in folder
-L, --list-files List files in folder
-m, --mkdir=DIRNAME Create a directory
-r, --rmdir=DIRNAME Remove a directory
-n, --num-files Display number of files
-p, --get-file=RANGE Get files given in range
-P, --get-all-files Get all files from folder
-t, --get-thumbnail=RANGE Get thumbnails given in range
-T, --get-all-thumbnails Get all thumbnails from folder
--get-metadata=RANGE Get metadata given in range
--get-all-metadata Get all metadata from folder
--upload-metadata=STRING Upload metadata for file
--get-raw-data=RANGE Get raw data given in range
--get-all-raw-data Get all raw data from folder
--get-audio-data=RANGE Get audio data given in range
--get-all-audio-data Get all audio data from folder
-d, --delete-file=RANGE Delete files given in range
-D, --delete-all-files Delete all files in folder (--no-recurse by default)
-u, --upload-file=FILENAME Upload a file to camera
--filename=FILENAME_PATTERN Specify a filename or filename pattern
-f, --folder=FOLDER Specify camera folder (default="/")
-R, --recurse Recursion (default for download)
--no-recurse No recursion (default for deletion)
--new Process new files only
--force-overwrite Overwrite files without asking
--skip-existing Skip existing files

The first thing we need to do with gphoto2 is to check that we can detect the make and model of our camera, if this part fails, then it is likely that your camera is not supported. If you have a particularly old DSLR (acutally, gphoto2 works with cameras other than DSLRs) then you might not get any support, or only limited support.
So for instance, my 12 ish year old Canon 300D is supported (yay) and here is what is supported:

Canon EOS 300D (normal mode)	Image Capture, Liveview, Configuration
Canon EOS 300D (PTP mode)

On the other hand, I would expect that any body I, or anyone else buys this year, will have to wait for some support to be added.
There is a list of supported models available at this url: http://gphoto.org/proj/libgphoto2/support.php