Depends on what you are referring to.

Increasing the contrast using an image editor software means making the darks darker and the brights brighter, and decreasing means the other way around. Increasing the contrast will cause loss of details since you won't be able to distinguish one tone from another as well as you could before you made them darker or brighter, and decreasing might give you more details, depending on your camera's or film's dynamic range and some other factors (I'll explain it later). On the other hand, increasing the contrast might make your images more vivid, whereas decreasing it might make them more dull:

On the image above, the middle one is the original, the left one is with decreased contrast (notice how it is more dull) and the one in the right has increased contrast (more vivid). You can clearly see a loss in detail on the one in the right, with increased contrast, whereas there really isn't any gain in details in the one in the left. Why? For two reasons:

1. Dynamic Range

   I used a cheap film to take that picture, it does not have a good dynamic range, i.e. there wasn't much of a difference between the darkest and the brightest tones to start with. The same is usually valid to cheap digital cameras.

2. The editing process (most important!)

   I edited that picture with an image editing software, after it was already processed and developed, which is the equivalent of editing a JPEG instead of a RAW if you shoot digital. What you are doing is editing what was somehow already edited by the lab guy or by the camera's software, it's like trying to change the colors of a painting that was already painted.

On the other hand, if your camera / film has a good dynamic range and you do your magic while developing the film or if you edit RAW, you can gain a lot of details by decreasing the contrast and messing around with some other lightning settings - the dynamic range is set by your camera/film, but you can mess around with it, paying the price for it, which is loss/gain in details for gain/loss in vivid tones. I can't develop the picture again, but I edited only certain areas of it to simulate it:

You can clearly see in the sky and buildings (brighter tones) that I gained a lot of details by decreasing the contrast and messing with some other lightning settings.