Transit visa is a kind of visa that is required just for stopping in any country without even visiting the country. If say, you are travelling from one country to another but you need to change your flight in a third country in between, a visa that is required just for stopping in such third country can be termed as transit visa. As far as the Indian Nationals are concerned the rules are different for different countries, while some countries allow a travel through without a visa, some require a visa for a travel through.
A visa for Singapore is not required if an Indian National will require a travel through Singapore without visiting the country. A visa free transit facility or VFTF is allowed for an Indian National only for 96 hours. Although certain other criteria also needs to be fulfilled in order to have the visa like a confirmed through air ticket or a valid passport or visa of the other countries, etc. But as for a primary fact the Indian Nationals are allowed to stop and travel through Singapore without a visa for 96 hours. But if the connecting flight or the stay requires more than a 96 hours stay then a proper visa will be required for so. Another very important criteria for a VFTF through Singapore for Indian Nationals is that they are allowed only once a VFTF in a journey if say the VFTF is already accessed in the forward journey then it will not be valid in the return journey. If a person requires to pass through Singapore twice in one single journey i.e. both for forward and return then such a person will require a transit visa for Singapore for one side journey.
But on the other hand a Canada visa application is required for an Indian National even if the person is just passing through Canada and the halt is even less than 24hours. For some countries Nationals if they are travelling from US a Canada visa application is not required but for Indians who are travelling from US and just halting at Canada for a flight change purpose then such will require a transit visa.
Getting a transit visa is a simple process although and can be well taken care of by the visa consultants without worrying you much but the process involves you and thus is a little troubling for the people.