Electro:
Electro is all about a driving drum machine beat with lots of cool synthesizer riffs, and a groovy bass line. The idea is to achieve a sci-fi sound in the track. Use heavy amounts of Distortion, delay, reverb, phase and chorus to produce robotic sounding vocals and instrumentation.
With just a few loops, and some good effects work you can make some really good Electro tracks. Using a vocoder or effects to get a Stephen Hawking type voice, for vocals is also a cool method.
Try to focus on your synth work. the rest of the instruments are really in the back seat to the synthesizer.
SynthPop:
If you've ever heard an early "MTV Era" band like Flock of Seagulls, a-ha, devo, and that crowd, then you've heard Synth Pop. Its very similar to Electro, except the focus is more typically "pop" rather than robotic, it is also; generally speaking, slower paced.
Devo had a more Punk mentality. Thomas Dolby was the first real 'geek' pop star. Synthpop uses all the classic instruments of pop and pop rock, but the synthesizer is the dominate instrument being used.
Consider 'I Ran So Far', it uses typical pop melodies, instruments and is paced as a pop song. Yet; if you pay attention, the song is driven by the simple synthesizer melody being played. 'Cars' by Gary Numan is probably the perfect example of a SynthPop track. The more robotic feel, processing on the instruments and emphasis on the synth is pure synthpop classic.
To make synthpop you should first listen to some 80's new wave groups, then try Savage Garden and jameriqaui. Although not classified as a SynthPop group jameriqaui uses the foundation of synth a great deal, which makes for good idea fodder.
To construct the tracks, keep the synth in the foreground with vocals. If you are any good, with a synthesizer you can substitute any, and all instruments with synth versions. Try using the synth in place of the lead guitar.
I will say its hard to make synthpop without being able to play a synthesizer well enough to do it all, or having a band to back you up. However with a bit of creativity, you can do anything with loops.
I suggest checking out Thomas Dolby and Gary Numan, they were and still are pioneers of electronica, that goes for anyone interested in making electronica songs.