The Diels-Alder reaction is a [4+2] cycloaddition reaction that always produces a six-membered ring and sometimes produces a bicyclic compound. It is a pericyclic reaction between an electron-rich conjugated diene and an electron-poor dienophile.
Diels-Alder reaction and mechanism
The Diels-Alder is a thermocyclic reaction between a conjugated diene and a dienophile. There are three arrows, and the important thing to remember is that they trace a circle in the same direction; that is, they either go clockwise or counterclockwise. I suggest picking one and sticking to it! Let’s check out the general mechanism
1,3-butadiene and ethene
Let’s look at this mechanism starting from the pi bond between carbons three and four. The pi bond attacks carbon 5 on the dienophile, which causes the pi bond between carbons five and 6 to attack carbon 1 in the dienophile, which causes the electrons in the pi bond between carbons 1 and 2 to move between carbons 2 and 3.
That’s a long-winded way to say that we end up with a six membered ring and our sigma bond between carbons 2 and 3 gets a pi bond. Of course, there is a transition state, so let’s see what it looks like:
Aromatic transition state
The transition state of the Diels-Alder reaction is 1) planar, 2) cyclic, 3) fully conjugated, and 4) has 6 pi electrons, which satisfies Huckel’s rule. That makes it aromatic!
Adding complexity
Bridged molecules
In our simplest Diels-Alder, we form a six-membered ring. But what happens when our diene is already a six-membered ring like 1,3-hexadiene? We actually form a bicyclic compound! The diene atoms that don’t actually participate in the form a bridge like this: 
Building bridges
Notice that we end up with a new six-membered ring and our original one basically folds out of the way. The red lines in the product are the new bonds formed from the red arrows.
Substituents
Ignoring stereochemistry for now, let’s check out how substituents affect the product. Let’s say we have 2-ethyl-3-methylcyclohexa-1,3-diene reacting with 1-butene. Depending on the molecules’ orientations, we can end up with two different products!
Regiospecificity
Notice that in the first reaction, the blue ethyl group is near the green methyl group but it’s different in the second reaction; the blue ethyl group is near the green ethyl group in the second reaction.
EDGs and EWGs
Electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) are often found in Diels-Alder reactions. EDGs (e.g. amines and alkoxy groups) are generally found on the diene, and EWGs (e.g. carbonyls and anhydrides) are generally found on the dienophile.
EDGs and EWGs
Of course, just like the ethyl group in the previous example, we could have two different molecules here. This is just to show there the groups are generally placed.
Heteroatoms and alkynes
Every once in a while you might come across an alkyne as a dienophile or a heteroatom in your diene. Let’s take a look at the reaction mechanism between a furan and ethyne as the dienophile:
Furan and acetylene
Notice that it’s not very different even though there’s a triple bond and oxygen? All we did was kick the oxygen up to the bridge and react with only one of the pi bonds in the ethyne; the other pi bond stayed there between carbons 5 and 6!
Endo and exo
Diels-Alder reactions can produce diastereomers called endo and exo products. If all groups are on the same side of the newly formed ring, it’s an endo product; if substituents are on opposite sides of the ring, it’s an exo product! Check out this image to see what I mean:
Endo and exo
