Disclaimer: Making explosives is very, very illegal in just about any context and location unless it's been pre-approved by the government. Besides that, attempting to make bombs is a very good way to end up bleeding to death, missing fingers, limbs or overall bodily cohesion, spending lots of quality time with police officers or otherwise having serious problems. Don't do it. If you decide to do it anyway, I assume no responsibility for any consequences of your own stupidity.

That said, let's get to giving Echelon something to munch on.

A pipe bomb is a simple form of homemade explosive device, consisting of a capped pipe of hard material containing both explosives (usually low explosive, such as gunpowder) and some kind of detonator. Something like this:

|=====|                  |=====|
|#####====================#####|
|##############################|
|#############################00000
|##############################|
|#####====================#####|
|=====|                  |=====| 

where the #s are the explosive, the =s and |s are the pipe and end caps, and the 0s are the fuse or detonator. Any form of (preferably reliable) fuse can be used as both timer and detonator; since the more exotic forms of high explosive are usually outside the means of your average homegrown terrorist, flame will usually suffice to ignite the explosive. Other detonators may be substituted for the fuse, but these are both (sometimes) more dangerous and (almost always) much more complex. Any form of hard, inert material may be added to the inside of the bomb for use as shrapnel.

Pipe bombs are popular with terrorists, criminals, and miscellaneous nutcases because of their simplicity, flexibility (you can make one with any size pipe, any amount of explosive, out of PVC so it doesn't set off metal detectors, with additional shrapnel, without additional shrapnel, et cetera), and shape (for various reasons having largely to do with concealment). In many of their common forms, however (for example, matchhead and black powder bombs), their manufacture, transportation, and use can be shockingly dangerous to the user, the user's friends and anyone else in the immediate vicinity (including but not limited to the intended victim). They can also be fairly recognizable as bombs, and the tools of their manufacture are highly distinctive; if you go to the hardware store and buy a few hundred dollars' worth of matches along with ten feet of pipe and twenty end caps, expect someone to take note.