My recommendation is to avoid all multi-drug "convenience" compounds; buy your cold/flu meds in single-drug forms and then just use what you need.
Why? Two main reasons. Firstly - why take drugs you don't need? Related to this - taking an antihistamine and an expectorant at the same time is like putting the accelerator and brake pedals to the floor at the same time - these two types of drugs have exactly opposite effects. Antihistamines try to "dry up" excessive excretions, expectorants try to liquefy gunk so you can cough it up.
Second - and perhaps more seriously - a lot of these multi-component products have acetaminophen in them. If you use more than one - or add more acetaminophen (Tylenol, etc.) on your own - you can easily hit the toxic level.
Yes, TOXIC. Way too many people think of acetaminophen as being completely harmless because it's unlikely to cause stomach upset. The problem is that taken to excess, it can destroy your liver - it should never be taken along with alcohol, for instance, or for a hangover. No one should consume more than 4g (4000mg) in one day - that's eight 500mg "Extra Strength" tablets. If you take those on top of a combo drug, you could seriously exceed the safe consumption limit.
So, here's a basic run-down:
Antipyretic/Analgesic: relieve fever/aches. Acetaminophen, aspirin, ibuprofen, etc.
Antihistamine: Relieve runny nose, hives, other allergic symptoms. Common ones are diphenhydramine ("Benadryl"), chlor- or brom-pheniramine maleate ("Chlor-Trimeton") or triprolidine ("Actifed"). All of these are sedating to one degree or another; loratadine ("Claritin") is the only non-sedating antihistamine available over the counter. Generics are often vastly less expensive than the name brand.
Decongestant: An oral decongestant works by constricting blood vessels, which can be a problem for people with high blood pressure or other vascular problems. Pseudoephedrine was the most commonly used such drug, but many states have restricted its sales due to the fact it can be used as a raw material to make methamphetamine. The unrestricted alternative is Phenylephrine; unfortunately, it's not yet available in a time release (12 or 24 hour) dosage form, and thus must be taken every 4 to 6 hours.
Expectorant: Guaifenesin increases secretion of water, thinning phlegm and making it easier to cough up. Note that this is the exact opposite of what an antihistamine does, so taking both drugs at once is paradoxical.
Alcohol: often used as a solvent and preservative in liquid medicines. It can amplify the sedative effect of antihistamines (probably intentionally, in the case of something like "Ny-Quil".) Best avoided for children and/or diabetics. Not wise to consume with acetaminophen.