TDMA and FDMA are one of the resource sharing implementations in the telecom network. Here's the explanation for each of them in simple way.
TDMA Time division multiple access TDMA is a method of communication. Here each user connected to the nearby tower gets specific time block for transmit and reception. The separation of the type of data transfer and congestion are handled differently in both 3G and 4G networks. Considering the networks are using digital transmission, the data and voice signals are lot clear and have less noise compared to old analog signals using same technology.
FDMA Frequency division multiple access FDMA is a method of communication where frequency slots are divided for each user on resource sharing model. It is one of the oldest method in the telecom communication. In earlier days the signal divided was analog hence had lossy transmission for audio and data. In modern days digital transmission has reduced the signal loss and hence improve voice and data communication in lossless way.
Do note that marketing people are promoting 3G and 4G as if it's some advanced technology. Underlying process for 3G however is still CDMA that makes use of either TDMA or FDMA or both. In case of 4G the networks make use of LTE that comprises of OFDMA and SC-FDMA. There is nothing ground breaking research on 2G, 3G, 4G as of yet, they are still making use of DMA (division multiple access) since analog days, only with difference of digitization that can serve larger number of users for a given band in specific tower region.