The Travel Agent and Tour Operators are normally two separate service providers handling different parts of journey, not always exclusive but the difference does lies in the service they provide.
From consumer point of view the big change happened in 1992, since then any one who is offering the services for travels are liable based upon their stake and expected profit which they are expecting. This is great development for the consumer side as now financial responsibly for the potential liability is divided into every party responsible.
The Tour Operators provide much more detailed services which you require from every little thing during the visit. It might not seem a huge difference but the both of these parties are at different end in case of any liability etc. As the travel agents don't have huge stakes to the whole journey, being just a go between, so they are taking commission for their services. Incase of the dissatisfaction or problem they might not be facing huge lawsuits as they are not the primary sellers.
The main difference between is that the scope. The Travel Agents provide the specific services unless one asks for them to handle other things, they act as go between the airlines and travelers. It is expected that the travel agents most of times have business inside their own office and don't have any stakes in the running of tours, or are not attached with the actual facilities and services.
The difference between Agent and Tour Operator can also be seen in the way they are being paid. The agents are given commission for their services for the normal things like air tickets etc. The bill of agent is often very small as compare to.
The services of both the tour operator and travel agents are essentially related so some of the organizations start to take both the activities. This however rarely occurs as the tasks of Tour Operators are huge as compare to the Travel Agent.