bands (short for pay bands) in FEDERAL (not contractor) competitive service vs. excepted service are slightly different.
Competitive service works as Band 1-4, followed by SES (senior executive service). With excepted service (which I expect most hacker type folks to be hired under) you don't have SES. You have Band 1-5, with Band 5 roughly = to an SES pay grade. Excepted services essentially means you are hired in for a special skill set, and you don't compete on the normal gov HR point system (which includes vet preference, disability, etc.). Excepted service tends to be used for hiring a specific person. Downside is that without competitive status, excepted service personnel can not move laterally in government.
in both cases, band 1-4 cover the same ground as GS 1-15, but with less stratification.
GS Grades go from 1-15, but each grade has 10 steps. GS 9 (average masters degree education starting point) will run 42K-56K base + whatever COLA (cost of living adjustment) you get for location. For Wash DC area COLA is +24.78%, bumping GS 9 to 53k (step 1)-69K (step 10). Each department/team is a little different, but most places I'm familiar with have a clear career path from GS7-GS13. GS14 and GS15 are more slot based, and generally are management positions.
Bands are tougher to move around in after your initial hire. It works out better for you if you just scrap into the next highest band, it works out worse if you land in the middle or the top of your band. Instead of step or grade based pay bumps, the band system is an "experiment" to incorporate pay for performance. Everyone gets their base pay (determined by band, and then further separated into high, medium, and low), and then there is an extra pool of money at the office level that is distributed by performance reviews. High performers get 1.5%-3% * (base pay + COLA), with low performers getting nothing. Without getting deeper into the weeds, most people can expect to get a ~1%-1.5% "bonus" annually. Theoretically, the bonus system is supposed to make up for the additional stratification of the Grade/Step system, but because of office politics the curve is pretty flat, and the high performers don't really see that much of a pay bump.
edited to add this is for FEDERAL employees. Contracting has a whole different set of issues and rules. In general, I'd recommend going Federal to get experience and a clearance, then transition to contractor status later in your career. Whereas most Fed salaries will top out at 160K-ish, contractor salaries w/ bonus can be much higher (2x-4x). Downside is stability and employment risk, and working for a client rather then being the client.
