Star formation is a process that has been studied extensively during the last decades. With the advent of advanced instruments our knowledge has expanded beyond the point of general descriptions. From simple models of gravitational contraction of spherical clouds we have managed to give more detailed descriptions about the inner processes and the chemistry within the environment of proto stars and young stellar objects. Still there are several issues that have not been fully understood, such as the formation of stars with very large masses, the exact mechanism that generates strong bipolar outflows and the chemical routes through which complex molecules are formed within the young stellar envelopes. A wide field of research is associated with observations of the rotational transitions of molecules in the centimetre and (sub)millimetre ranges. Our study concentrates on the analysis of data from two spectral surveys one in the centimetre and one in the millimetre range. We have performed calculations of column densities and abundances of several species from four different sources of different masses and evolutionary stages. The sample includes two high-mass young stellar objects (AFGL 2591 and lRAS 20126), one low-mass binary system (NGC1333-IRAS 4), and a well known photon dominated region (Orion Bar). For AFGL 2591 temperature and density profiles are developed based on a combination of calculations and non-LTE radiative transfer modelling of spectral lines. The same analysis revealed great abundance enhancements for CH30H, and S02 of factors ≥103 suggesting the existence of a hot core. Related to this is the detection of CH30CH3, the first complex organic molecule detected towards this source. Furthermore, CH30H maps have revealed an unusual ring-like structure which has been explained by a scenario that requires the enhancement of CH30H in outflow regions of shocked gas. The study of the radio recombination lines in combination with radio flux measurements leads to the conclusion that the large radio source in the vicinity of the AFGL 2591 is an ultra compact HII-region associated with a different object in the background. For lRAS 20126 and IRAS 4 the conditions of the outer layers of their envelopes are estimated by non-LTE radiative transfer modelling of mainly centimetre lines. The results are about T≈30K and nH2≈105cm-3 typical in the outer regions of young stellar envelopes. Inverse P-Cygni profiles of H2CO at 14.488GHz, have been detected which imply kinematics of infall within the envelopes of both sources. This suggests that the sources are at an early evolutionary stage. The profile is more prominent towards lRAS 4, which is a complicated system of more than one component. We suggest that motions of infall might exist in outer layers of the surrounding cloud apart from the regions near the stellar components. Finally the maps of the Orion Bar have confirmed the clumpy medium by the detection of two separate clumps in the region. Their masses are estimated to be about IIM®. Based on radio recombination lines the temperature of the Orion nebula has been found to be 9017 K, reflecting a mature HIIregion. The findings of this study will be combined with the JCMT Spectral Legacy Survey, which is currently collecting data from observations in the submillimetre range towards the same sources.