This can be achieved by vertical spacing, or by indentation at the beginning of paragraphs. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
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The only style with indented paragraphs is known as "the modified-block style with indented paragraphs." In it, the date, closing, and signature block begin at the center, as in the modified-block style.
However, each paragraph is indented a half-inch (1.27 cm).
Both of those would be indented as a block, though, not just the first line, like this: This is an example of a paragraph that is block indented.
The whole paragraph is indented, not just the first line. Paragraphs are not indented in the modified-block style.
Indents were (and to some extent still are) much more common in printed material; this may be due to space restrictions and the cost of paper.
Most articles I read on the internet - even very formal ones - use gaps to delimit paragraphs, and this does not strike me as "wrong" or "informal" in any way.
Often if you're writing for a publisher, newspaper or magazine, there will be a house style that will dictate the layout.
If you're writing for yourself you're free to choose what you prefer.