There are many ways in which water systems can be broadly classified. One of these is as freshwater and estuarine sytems. Freshwater systems are those that have water not containing salt which include lakes and rivers. Freshwater systems can be further classified into lentic and lotic water systems. The former consists of small water bodies where the water does not flow. This can range in size from small puddles to large lakes. The latter i.e. lotic systems are systems of moving fresh water. This includes springs and rivers.
The statement in the question that lotic systems have a more uniform temperature is not very accurate. For instance, the water in a river that originates from a glacier situated on a mountain and moves towards the sea is going to have a wide range of values along the path followed by the river. If the temperature of water at a particular location across the width of a river is measured the values would probably turn out to be relatively similar. The uniformity in temperature is due to the fact that the water is moving and not stationary, unlike that in a lentic system.
"Lotic" refers to rapidly-moving water. The reasons for uniform temperature have to do with the fundamentals of thermodynamics and their interaction with a rapidly-moving environment.
Any time a body of water is exposed to the sun, energy from the sun will interact with the water and raise its temperature. In a stagnant (lentic) body of water, the water molecules closest to the surface will remain on the surface for an indefinite amount of time, absorbing more and more solar energy, while the water molecules beneath them are shielded by those above. This creates a temperature gradient with cooler molecules on the bottom and hot ones on the top.
To visualize this, simply imagine a glass of water with a blowtorch applied to the surface. Now, to transition from a lentic to a lotic environment, all we have to do is physically mix the water. In a lentic environment, the rapid flow of water creates turbulence, literally mixing the water molecules around, so that they spend no more or less time at the "top" or the "bottom" of the water body. This means that the solar energy is distributed more or less evenly among all the molecules in the system.