There are actually two kinds of fibre and their actions are rather different. Soluble fibre - the kind found in oatmeal or inside apples or other fruits - likes to absorb water and turn into a sticky, gummy kind of paste. In doing this, it's transit time through your system is slowed down and it actually captures and traps bad cholesterol, stopping it from going into your bloodstream. This contributes to it's cholesterol-lowering and heart-protecting effect. Then there is insoluble fibre - this is the kind lovingly referred to as "roughage", and for good reason. Insoluble fibre wants nothing to do with water - it just wants to get away and get out! In doing so, it sweeps your colon, turning over cells and stimulating new cell growth, which not only keeps you regular, but also helps prevent colon cancer by removing toxins and preventing cell buildup.
You can find your good friend fibre in fruits and veggies, whole grains, cereals (Fibre 1 Crunchy original has 14g fibre per 1/2 cup - half of your daily requirement - with only 100 calories and no sugar!), beans, nuts, seeds and lots of fortified foods, where fibre is added in. The great thing about getting it naturally, though, is that you also get lots of antioxidants and other important health-boosting nutrients along for the ride! Just be sure to drink plenty of water when eating lots of fibre. But I will give a word of caution: If your diet is low in fibre, it's not the best idea to load up on your 25-30g daily requirement right away - introduce a little more each day and you will reap the benefits, and quite possibly slim down a bit too!