Got $1500 to spend:::

[Analog6]

But do cameras have a certain life for how many photos they can take?

Yes, as David says, there is a finite life. It is a bit tied to the numbers, so see below.

I just see that on some of the used cameras, they mention how many photos have been taken in the life of the camera?

This is so buyers can make an informed decision.

Newer is not always better?

Again due to the numbers, see below.

So, a 40D from two years ago might be better than a brand new 550D??

In the way that it might have amore features that you want and a more robust build. It does all come down to individual wants and needs. Do try to go to a camera store and ask to handle the various models and shoot a few frames, that is the best way to get a feel for what will suit you.

The three series of numbers used by Canon (and all are good cameras):
1. ###X (eg 500D) are the entry level ordinary consumer cameras. Lightweight, easy to use, basic features (but lots of them). Crop sensor

2. ##X (eg 40D, 50D) are the Prosumer cameras, for the advanced amateur photographers who want more features and a more robust build. Crop sensor

3. #X: (eg 5D, 7D, 1D series) These are the professional bodies with the top of the line (and therefore most expensive of course) being the 1D series. Weatherproofed, heavier, more robust build. More features. Bigger sensors up to full frame.

Sensor size is based on the old 35mm film format, cameras range from APS sensors (approx 1/2 that size) through various crop factors from 1.6 to 1.3, then to full frame. This crop factor affects the lenses you use.

For starting out, a crop factor camera is still a very good camera, so don't do your head in too much over it.

And yes, it is all awfully confusing and there is a heap to learn.