The decimal point is the dot
separating the integer
parts of a number expressed in decimal
notation, as in 3.14
It is usually written with an ordinary full stop (or period), ASCII 46, but a mid-height dot (· or ·) is sometimes used, as in 3·14 - the disadvantage of this being that the same sign is used for multiplication.
It is usually spoken as "point", as in "three point one four", though other words such as "spot" and "decimal" are occasionally heard.
The same marker is used for non-decimal notations, as in binary 1.1 for decimal 1.5 though any non-decimal use is going to be very rare.
In everyday use in continental Europe a comma is used instead, as in 3,14
This makes for confusion (if I may diverge into decimal notation generally), since they also commonly use a full stop as a thousands separator, so 10.000 for ten thousand, which in English-speaking countries is traditionally notated 10,000 with a comma. The scientific standard is to write 10 000 with a thousands space. This space convention is widespread in scientific use, and was mandated by the Ninth Conférence Générale des Poids et Mesures in 1948, and reiterated in the 2001 CGPM. The Ninth CGPM also affirmed that, because both are in widespread use in different language communities, for the decimal marker you may use either the full stop or the comma, as in 10 378,15
The current use of the point is attributed to John Napier, from 1617, the year of his death, or a little earlier; though a large variety of other methods were used in the preceding thirty years since Simon Stevin introduced decimal notation into European mathematics in 1585, and it appears (I'm still not sure from looking at several sources, see below) that someone else used the point in 1593, before Napier.
Stevin first used the notation 3 (0) 1 (1) 4 (2) in his book De Thiende, published in English as The Art of Dime (i.e. of Tenths). Other notations tried included 3(14 by Kepler, 314 by Henry Briggs, and 3|14 by Francois Viete.
The second of these gives some other interesting dates for notation: fractional line 1220, equals sign 1557, plus and minus in 1600, less-than and greater-than in 1631.