One terabyte (TB) is about 1000 gigabytes, or roughly 1 trillion bytes. You can buy 4 TB hard drives today, so we are beginning the time when this term comes in to common use. Gigabyte used to be an exotic term too, until Moore's law made it common.
One gigahertz is 1 billion cycles per second (a megahertz is a million cycles per second). Gigahertz is a measure of speed, very roughly the rate that at a CPU can do its simplest operation per second. Gigahertz does not precisely tell you how quickly a CPU gets work done, but it is roughly correlated. Higher gigahertz CPUs also tend to be more expensive to produce and they use more power (and as a result give off more heat) - a challenge for putting fast CPUs in small devices like phones. The ARM company is famous for producing chips that are very productive with minimal power and heat. Almost all cell phones currently use ARM CPUs.
You should be comfortable doing simple arithmetic to figure MB / GB sizes, just as you should be able to do basic computations with second, miles, kilos and so forth.
Basic plan: before adding measures X and Y, convert them to be in the same units.
Alternate Terminology: Kibibyte Mebibyte Gibibyte Tebibyte
It's convenient within the computer to organize things in groups of powers of 2. For example, 210 is 1024, and so a program might group 1024 items together, as a sort of "round" number of things within the computer. The term "kilobyte" above refers to this group size of 1024 things. However, people also group things by thousands -- 1 thousand or 1 million items.
There's this problem with the word "megabyte": does it mean 1024 * 1024 bytes, i.e. 220, which is 1,048,576, or does it mean exactly 1 million, 1000 * 1000. It's just a 5% difference, but marketers tend to prefer the 1 million interpretation, since it makes their hard drives etc. appear to hold a little bit more. Also, the difference grows larger and larger for the gigabyte and terabyte sizes. In an attempt to fix this, the terms "kibibyte" "mebibyte" "gibibyte" "tebibyte" have been introduced to specifically mean the 1024 based units (see the Wikipedia article on kibibytes). These terms do not seem to have caught on very strongly thus far. If nothing else, remember that terms like "megabyte" have this little wiggle room in them between the 1024 and 1000 based meanings. We will never grade off for this distinction. "About a million" will be our close-enough interpretation for "megabyte".
