Do to the legal status of cannabis or 'weed' in much of the would, it is very difficult to buy weed online. Searching the internet for phrases like 'buy pot online' and 'buy weed online', is not going to lead you to lead you to any sites selling 'real' weed.
While it is well known that cannabis seeds are sold online with some frequency, due to lax enforcement or laws in some countries, the same is not true for actually cannabis buds. There are some locations, deep in the darkest corners of the Internet, where it is possible to purchase cannabis, however, these are very difficult to find, require technical expertise to access, and may rip you off, since the transactions are untraceable and non-reversible.
Cannabis is a psychoactive plant, with many effects. These effects are not unique to cannabis, and can be found in other plants. Some people who seek the effects of cannabis, may attempt to use a legal herb or combination of legal herbs to find what they are looking for. Since these herbs are legal, they are commonly sold online.