Brother 'Aziz, Riba can not be totally explained in a single sentence, but the most basic translation of it is Usurious Gain.
The most well known kind of Riba today, is the type in which side A loans a given amount of money to side B, and side A demands that side B pay back the debt PLUS something else. Even if it was just a single dollar that side A asked to be payed back on top of the true debt, that would be Riba, one of the greatest sins. It doesn't have to be money that side A asks for extra; it could be something else, and that would still be Riba. For example, side A loans $500 to side B; side A then says "I want back $500 and also to be able to use your car for a whole week". That is Riba, usurious gain. In either case, it is the same whether it is done in a single payment or in installments; it is still Riba.
However, as Ladyrain mentions, there are other types of Riba which are not so well known amongst the Muslims, although the judgment remains that falling into any of them is considered a major sin. An example of this is selling pure gold for pure gold, while the weights are not equal. For example, side A has a gold bracelet, and side B has a gold ring - the bracelet weighs more than the ring - in that case, it is Riba to exchange them in a buying/selling deal.
There are other types of Riba, so the one who wishes to get involved in trade transactions must seek the obligatory knowledge of the Religion pertaining to those matters, so that he does not fall into the numerous sins that many ignorant Muslims do today as a result of not having studied what Allah obliged them to learn.
