International trade is the exchange of goods and services between two (or more) countries. When goods (services) are brought in, it is called import and when goods are carried out its called export.
International trade is necessary, because the scare resources are distributed unevenly between different countries and thus some countries are better producing some products than other. For example Zimbabwe has virtually all the world's chrome resources, so it has an advantage of extracting it while the workforce in Germany can produce chrome tapes (because they have the capital and technology). Without international trade the chrome tape production would not be possible (and thus the quality of sound suffers) etc. Furthermore, the Zimbabwe itself would have no remarkable benefits from its chrome resources as the domestic market for tapes is probably very small and the building of tape factories is expensive.
Another, not so obvious and maybe arguable, uneven allocation is the allocation of education for labour. As the possibilities for getting education in "west" are much greater, the services sector (or tertiary sector) is much more developed (e.g. workers in banking sector have to be trained for long time). In the developing countries the educational standards are lower, thus they have developed manufacturing and extraction (primary and secondary) industries, which require much not so educated labour.
The international trade helps to make the distribution of resources more even. The countries can specialise to work that they are best at. It also helps countries to obtain the products they otherwise might not get. Also, it increases the variety of goods. E.g. UK can produce cheese, but with international trade English are also able to enjoy French cheese etc.
The specialisation itself can create disadvantages for trade. If a country is too dependant of one industry and the prices fall in that industry, then massive unemployment will occur. This was the case in...