the other guys comical answers for a reason .... your question is so unspecific that its really difficult to give good answers
what is your electronics background ?
have you already started diploma, BSc etc courses ?
what electronics fields interest you ?
Dave
yeah, there is a lot of stuff in electronics:
digital electronics, which are low-power and low-voltage, very sensitive to timing issues, moderately sensitive to noise; a lot of this involves working with ICs. here, things like component switching times, propagation delays, ... may become significant factors. say, one has to make sure that the data bits reach the target before the clock pulse rises or falls, as otherwise the output may turn to garbage.
analog signal-processing electronics, where typically noise and precision are much bigger factors (things like noise, or component values being off, can have a significant impact on correct behavior of a circuit). one may be left considering the values and tolerances of their resistors and capacitors and similar (with resistor values that would be absurdly large in other contexts, such as needing 700kOhm for a circuit to work, but it will fail if below 675k or over 750k).
then there may be things, like dealing with signal attenuation over distances, or how long it may take something to travel that distance, ...
power electronics, where things like wire gauge, component power-ratings, and heat, become significant factors (is the wire thick enough, will this component handle this load or fry, ...). types of issues become things like, say: wires overheating due to current (melting insulation is bad), transistors overheating and/or catastrophically failing, issues due to the inductive kick from switching large coils (big white sparks and the smell of ozone aren't necessarily good, nor necessarily when these sparks blow holes in metal or ones' power transistors explode as a result), ...
then, there are subsets of power electronics, like for example, considering specifics of winding motors (wire gauge, whether to wind the coils in series or parallel, ...). and motors may be small DC motors using permanent magnets, and larger motors using field-coils in place of magnets, or induction motors using "squirrel cage" rotors, ...
likewise, to one person, "battery powered" may imply some AAs or maybe some LiON or LiPo cells (with low-voltage DC power), and "mobile" means "fits in someones' pocket". but another may well think of "battery powered" in terms of a banks of lead-acid batteries (say, at anywhere from 24 to 144 volts), and "mobile" as something mounted in the back of a pickup truck or otherwise rolling around on wheels.
and, one person may think in terms of mW, and the other of kW.
and, some projects may involve multiple types at the same time.
say, some parts are digital and analog electronics, and a lot of other parts are power electronics.
nevermind parts which overlap with mechanical systems, like gearing, solenoids, and interfacing electronics with hydraulics and pneumatics. a project may be partly software, partly electronic, partly mechanical, and partly pneumatic (at kW power-levels and above, pneumatic and hydraulic start to look a little more attractive from a component price perspective).