Whether or not you can say that a capacitor can be in the "microfarad" range depends upon your country's convention for cap values. In the U.S., we idiots refuse to use nanofarads and millifarads, stubbornly sticking with picofarads and microfarads only. So, our values for ceramic caps can range from 0.001µF (1000pF or 1nF) to around 0.68µF (680,000pF or 680nF). So, a lot of the answer lies in your perspective.
As Nigel says, it should be obvious to the most casual observer whether a cap is microfarads or picofarads. But folks new to electronics are hardly "casual observers" since they're still on the flat slope of the learning curve.
Aluminum electrolytic caps are usually characterized by an aluminum can with a crimped circumference around one end, whether it has axial or radial leads. They tend to have values of 1µF or greater, although I have seen a few in rare instances as small as 0.22µF (220nF). The sky is the limit for an aluminum electrolytic's value, although they tend to ease into the millifarad catagory as their value equals or exceeds 1000µF, at least in technologically progressive countries. As timing components for 555 timers, they aren't the best for precision work as they tend to have tolerances of +80%/-20% and the larger ones have lots of leakage resistance which really screws up the timing equations.
Tantalum electrolytics tend to be "conformal coated" or "dipped", looking more like blobs. They tend to have values ranging from just under 1µF to as much as 470µF, although it isn't common to find them much over 47µF these days. They originally had their values marked on them as a color code and tantalums were some of the prettiest components in electronics. But these days, they're marked just as any other cap is. Although they have better tolerances than their aluminum brothers and a lot lower leakage (higher leakage resistance), they don't have the best temperature coefficient in the world.
Polyester (Mylar) caps tend to have values ranging from around 0.001µF (1nF) to 0.47µF (470nF). They are non-polarized and great for most audio work but usually horrible for RF work because of their wound construction. For your 555 timer work, they are usually the best choice for they have a low temperature coefficient and have good tolerances, usually available in 10% and 5% tolerances but sometimes found with 2% and 1% tolerances.
Ceramic capacitors range in value from around 1pF to as high as 0.47µF (470nF) although values much over 0.1µF aren't very common. They shine as RF bypass capacitors, but suck as timing components because they have some of the most awful temperature coefficients around. Of course, that could put them into a use in a temperature-to-frequency converter! Ceramic capacitors in low pF values are available with a nearly zero temperature coefficient for frequency-sensitive/temperature-sensitive work. They're called NP0 (that's NP-zero, not NP-oh) ceramics and will usually have the edge opposite the leads marked with a black sploch.
Dean