Progressive Metal

A modern genre, just recently being created from Progressive rock bands such as Rush and Queensryche.  Prog metal stands out from other genres by performing extremely difficult musical feats.  This is some of the most technical metal you will hear.  Insane timing changes, complicated drum beats and guitar work that will disturb you with the sheer difficulty of it are musts.  Almost all Prog metal bands will use keyboards to great extent, even playing solos on them.
Prog metal often seems not to flow, because of the constant shifts in music styles and tempos.  Mellow and melodic sections of songs are quite common, though the music is always technical.  Extremely long songs are another trademark of the genre.  Both Dream Theatre and Symphony X have epic songs that measure up to over 20 minutes in length.
Often you will also see instrumental songs my Prog metal band.
Progressive metal has a wide range of style, so vocals can be nearly any style.  Take for example, the Swedish group Opeth, who employ death metal vocals and very deep distortion.
Lyrically, this genre deals often with emotions and new-age thoughts and ideas. 
Concept albums, (an album based around a single story or idea), are trademarks of the genre.
Some people consider Prog bands to be showoffs, but when it comes down to it, I think they are just jealous. 
I RECOMMEND...

DREAM THEATRE-
A great band hailing from the USA.  All members of this band are amazing!  Their songs range from ultra-heavy to soft and melodic.
High, clean vocals and amazing drums.

For premium enjoyment: check out

- Six Degrees of Inner Turbulence - A lengthy 2-disc affair(featuring The Glass Prison- best song by Dream Theater). The second disc is one huge 42 minute story.

- Metropolis Part 2: Scenes from a Memory- A concept album about a murder mystery, with unexpected twists. (Listen to the song Home, which uses the traditional Indian instrument the
sittar)

-Images and Words: An older album, but no less great.  Some awesome, very heavy tracks (Pull me Under) and one of my favorite all time tunes; Metropolis

SYMHPHONY X-
If Dream Theatre is 50/50 Prog/Metal, Symphony X is 70/30 Metal/Prog.  Almost all the tracks I have heard by Symphony X are heavy, rocking tunes.

-The Odyssey- A great concept album regarding the Greek epic of the same name.  Check out: Inferno, King of Terrors (great riff) and the epic 24-or-so minute 'The Odyssey'.

- V: The New Mythology Suite
I have yet to hear the entire album! More info to come!  It is held in high esteem by many!