Most individuals interested in getting fit focus on "losing weight". But, really, it's not the amount of total pounds you weigh that needs to be focused on. It's your body composition (percentage of body fat). It's not a good idea to watch the scale, because your body weight fluctuates so much and is skewed by so many factors (hydration levels, whether you've gone to the restroom or not, etc). A better test is to regularly monitor your girth, which requires you to use a measuring tape and measure around a few of your body areas. If you're losing body fat with an exercise regime and healthy eating habits, it will show in a change in your body girth. An important thing to note is to make sure you test the same exact areas every time. A suggestion of areas to measure are: 1) around your waist at the belly button, 2) 6 inches above your knee cap at your thigh, 3) around the nipple line.
Although, you forgot to mention that muscle weighs more than fat, which is why sometimes you won't appear to be "losing weight" when you weight train...because while you are losing fat, you are "gaining" that heavier but more healthy muscle tissue. I don't measure my girth, however. At my age, I am trying to appreciate the concept of a "Buddha Belly" a bit more. I am in good shape but I don't have rock hard abs...and I finally decided that the stress and effort that would go into maintaining those at my age would not be worth it. A little softness isn't a bad thing.