For autosomal DNA, segments are measured 3 ways:
base pairs (bp) – these are the individual building blocks (molecules) that form each chromosome. Over the entire set of chromosomes there are 3.2 billion base pairs. Each chromosome has from 48 to 250 million base pairs. So a segment can be defined by the Start Location and End Location. Think of base pairs as a physical picture of a segment – it’s physical length and location on a chromosome.
Example: Start at 23,500,000; End at 46,300,000
Many of us round to the closest Megabasepair (Mbp). A Mbp is 1,000,000 bp
Example: The segment is 23.5-46.3Mbp
Rounding makes these numbers much easier to read and to type. And, in my opinion, there is virtually nothing lost in accuracy. Mbp is just fine for genealogy, triangulation and chromosome mapping. If you want to do some analysis of a particular part of a segment for some scientific or medical reason, you may want to use bp. (I’ll discuss “fuzzy data” and “fuzzy segments” in a separate blog post)
centiMorgans (cM) – I think of cM as a “quality” factor, or a “genetic distance” of sorts. The cM is the best measure we have of genetic distance, but it is far from perfect. The cM is empirically derived – that is scientists have recorded many observations and put them into tables. From these tables the cM value between any two points on a chromosome (as measured by bp) can be determined. In very general terms, more is better, and the larger the cM of a shared segment, the closer the Match would be. DNA is very random, and there are wide ranges of cM vs cousinship (including much overlap). See these references for more info ,  and .
Example: A segment may be 15.4cM
SNPs – the single molecules (nucleotides), or base pairs, which show some amount of variation in human DNA. Most (99%) of our DNA is the same. For genealogy, we are looking for SNPs (sometimes referred to as markers) which are known to vary. The difference in our SNPs is what sets us apart. Basically each SNP can have one of 4 values: A, C, G or T. Each of the autosomal DNA testing companies uses a slightly different “chip” to determine these values, and they each effectively test a different number of SNPs – usually in the range of 600,000 to 700,000 SNPs. These are spread out over all of your chromosomes – think of them as a sampling of your DNA (a sampling of the most variable parts of your DNA). This might range from about 10,000 SNPs on the smallest chromosomes to over 58,000 on Chromosome 1.
Example: A segment may include 2,451 SNPs
Note that there is no firm correlation between these measurements. We can convert temperature measured in Centigrade to Fahrenheit because they both measure the same thing. All of the above measurements, measure different things. However, on average, there are about 100 cM for 100 Mbp
So, in summary:
A segment may be described: Chr 6: 23,500,000 to 46,300,000; 15.4cM; 2451 SNPs
A short cut description may be 6: 23.5-46.3Mbp; 15.4cM; 2451 SNPs
03 Segmentology: Measuring Segments; by Jim Bartlett 20150513