Colon means the same in algebra as it does in arithmetic, that is, it is usually used to express a ratio.
3:8 usually means the ratio of 3 to 8; it can also mean ⅜ expressed as a fraction.
a:b usually means the ratio of a to b and it can also be expressed as a fraction a/b.
The semi-colon (;) is used in some countries instead of the comma as a separator between coordinates or ordered pairs, etc., for example, (2,3) can sometimes be written (2;3).
Sometimes it can be specifically defined to mean something different. So in the example you have given semi-colon seems to be defined as the addition operator "add" (+), so 3+4 in this special case is defined to be the same as 3;4.