The Storyline
The storyline of Digimon:Digital Monsters is that seven kids at a camp get these little devices and are sucked into another world, the world of Digimon. They land on a huge island called File Island and each befriend a creature called a Digimon. Later on they find out that their digimon digivolve (which means they advance to the next level and grow bigger), when the kids are in trouble. Their Digivices have the power to digivolve their digimon as well as drive out and destroy the Black Gears (see below). The kids find out that they are the Digi-Destined, a group that has come to the Digi-World to save it from evil. Until they destroy the evil they will not be able to get back home. During their adventures the kids learn that the Earth and the Digi-World are connected together because both of the worlds' digital networks are the same. So whatever happens in the Digi-World will also affect Earth. They also find out that alot of good Digimon are turning evil because of the Black Gears. They find that these Gears are coming from Infinity Mountain and are controlled by the Champion evil Digimon Devimon. After defeating Devimon they get a message from a person called Gennai to go to the Continent of Server and save it from another evil Digimon and to find Tags and Crests that allow their digimon to digivolve into Ultimate level.
When they get to Server, the first bad Digimon they encounter is a rock-and-roll Champion Digimon known as Etemon. Everyone eventually finds their own Tags and Crests and defeat Etemon (with Datamon's help) but Tai and Metal Greymon get sucked into the resulting vortex. They come out of the portal and land back on Earth. Tai and Koromon go to Tai's house and meet Tai's sister Kari, who somehow knows about and can see the Digimon on Earth. Tai also finds out that very little time has passed in the real world and that it's the same day he left for the camp. (They had been in the Digi-World for ~4 months).
Tai goes back to the Digi-World but finds out that all the other Digi-Destined have separated to go find him (Tai had been gone for about 2 months). First Sora and Biyomon left the group, then all the others left until only T.K. was left. Tai found T.K. and learned that a bad bat Digimon named Demidevimon was trying to stop the Digi-Destined abd get the Crests. So far, his plans have been brilliant but always back-fire due to some mistake or (mostly) to Sora's intervention. We also learn that he works for the next bad boss Myotismon. Later, Myotismon learns that another child will be joining the group from Earth. After much research he eventually finds and opens a gate to the human realm but the Digi-Destined are right behind him! But who will get to the Eigth Digi-Destined first? And who is he or she? Only time will tell...
ITEMS
DIGIVICES:
Every Digi-Destined needs one of these. Their Digivices are the
gadgets that make their Digimon digivolve and more. They appear
in every episode. The Digivices were first seen in Episode I
"And so it Begins" when they fell out of the sky and
the kids grabbed them. Then they fell into the Digi-World and you
know the rest...The Digivices have several uses: 1) They make
their Digimon able to digivolve when they are in danger. Light
comes out of it and whoosh a totally new Digimon! 2) A high-tech
compass and locator. Beeps when danger is near and helps the
group find each other when separated. 3) They kill Black Gears.
Hold it up to a Black Gear and it will zap it right out and you
get lotsa fireworks. 4) With Izzy's laptop they can a)Be used to
communicate ("A Clue from the Digi-Past") and b) Store
Digimon information which can then be downloaded into Izzy's
computer ("It's all in the Cards.") SO...It's a good
thing batteries came included with these nifty devices.
BLACK GEARS: The Black Gears are little evil gears that were controlled by Devimon, we do not know if he made them or not. The Gears twirl around the normally good Digimon and turn them to evil as well as make them Devimon's slaves. There is no known limit on how many Gears you can have in your body. It's just like Steroids, they cause you to get stronger and meaner but once they're gone you get normal again. (like Leomon). The gears have stopped appearing when Devimon was defeated, but a Whamon with a Black Gear in him was discovered in "Journey to a New Continent" so there may be other Gears still out there. Devimon used to control the Gears from Infinity Mountain and he sent them from there as well. They apparently soar through the air like UFO's-that is, according to Izzy. (Ep.4 Biyomon gets Firepower). Well, if you happen to have a Digivice, getting rid of a Gear is easy. If you don't then you're out of luck unless you can knock it out of your body. The Digivice way is just to touch it to the Gear and it will pop out and explode. We never find out how the Black Gears work so maybe we'll never know.
Tags and
Crests: The Crests are what makes the Digivolved Digimon able
to digivolve to Ultimate level. All the Tags do is act as a
holder necklace for the Crests. So far all the kids have found
both their Tags and Crests but not all of them have yet
digivloved into Ultimate level. For it to work, the Digivice's
light shines up and goes through the Crest. After most Ultimate
Digivolutions, they play the "Hey Digimon" music.
TAI- Crest of Courage 
SORA- Crest of Love 
MATT- Crest of Friendship 
IZZY- Crest of Knowledge 
MIMI- Crest of Sincerity 
JOE- Crest of Reliability 
T.K.- Crest of Hope 
KARI- Crest of Light 
Shown with Tag as
well.