Fair enough, if you don't know what an erection is in the most recurrent modern usage, it's when the penis swells up when sexually aroused (or, as LeoDV rightly mentions, in the case of morning wood, for other reasons); blood flows into the penis's erectile tissue. It is then blocked off by a valve of some description so (hopefully) it stays hard.
The obvious question would also be, what's the average erection length? Well, I'm no authority on this myself, but I'm told (by Wick's writeup on penis size) that that depends on which study you go for, and it's normally between 5.5 and 6.5 inches. Needless to say, I myself am a firm believer in the credo that "it's not how much you have but what you do with it" simply because logic dictates that, if one had a yard in the very literal sense of the word (3 feet!) but could only keep it up for 45 seconds, I doubt that it would please one's lady friend too much. (There will, of course, be exceptions to this.)
Another interest factoid about erect penises is that not all of them are perfectly straight, in fact, barely half of them are. We have the stereotypical image that, like a sword, rifle, or any other weapon, one's organ is perfectly straight edged. This is true in a majority of cases, according to the Journal of Sex & Marital Therapy, Vol. 23 (1997), in which penises, their lengths, angles, and curvatures were measured from a photographic sample of around 500 or so participants, but that study also shows 13.6% of erections lean to the left slightly; the reason for this, they hypothesise, is that, given that most people are right handed, they tend to push their penises over while masturbating. Given the much lower proportion of left-handers, the 1.2% of penises which, when erect, curve towards the right appear to support this theory. Kinsey's work appears to support this; he gauged 87.3% as being "straight" erections, 9.5% as being left-curves and the remaining 2.4% as right curves.
Erection curves also persist in the y-axis as well. The above study (and Kinsey) show (63%, or 70.1% according to Kinsey), with the proportion of u-curves (penises that bend upwards slightly, and often have a high angle of erection) at 22.2% (Kinsey: 24.9%) and n-curves (penises that bend downwards slightly, although one n-curve erection in the 1997 study was bent right over alarmingly like a horseshoe!) making up the remainder. Here the more recent 1997 study departs from Kinsey's measurements, giving a proportion of 14.8% - almost thrice as much as Kinsey's research shows.
Now although there is well over a two-thirds majority in straightness of penile erections in both axes, quite often an erection that is straight in one axis will be curved in another. Thus, a stereotypical ramrod-straight erection (as is often implied in pornos and rather risqué romance novels) only accounts for 54.3%. Indeed, given various aspects of female anatomy, specifically the locations of the g-spot and clitoris, I would have to say that, quite possibly, a u-curve erection would probably be most exciting for one's lady friend, although as far as I know, no studies have as of yet been carried out on penis shape and women's preferences with regard to them. (I may, of course, be wrong, if you know of one, /msg me.)
Thus, I believe it fairly safe to say that erection shape really is of no real consequence - unless, of course, like the n-curved individual in the 1997 study, it's bent almost double!
(Incidentally, Wolfkeeper says this: "I've found it's possible to train erections- the valves at the bottom have muscles around them, you can train them up, and change the angle of the dangle, you do it by moving the (semi)erect penis around and increasing the pressure on them. To move it left, push it gently right until you can feel the stretch. It's only a small effect though; it probably doesn't turn a curly whirly into a ramrod.")
Here follows a rundown of synonyms/terms for an erection, some common, some less usual. You may have heard some, all, or none of these, but I digress:
Angry in the pants
Baloney pony (thank you Junkill)
Cheeky monkey (that needs to be spanked)
Chub (thank you Junkill)
Cockstand (James Joyce's love letters to Nora Barnacle use this one lots.)
Cracking a fat (Australian, thanks La petite mort)
Engorged flesh (popular in bodice-rippers)
Evidence of arousal
Intercuntinental ballistic missile
Loaded for bear
Manhood (NEVER use this word. Ever. It's truly vulgar and clichéd.)
Pocket rocket (thank you GrouchyOldMan)
Rock hard cock
Smiling in one's trousers (thank you Wertperch)
Stand to attention
Trouser grin (thank you Wertperch)
Tube steak (thank you GrouchyOldMan)
Turgescence (little-used derivative of the French erectile term "turgescent")
Yard (Hopefully a metaphor!)
If I've forgotten any obvious ones, don't hesitate to /msg.
Source - www.erectionphotos.com