How to highlight a particular line in emacs?

You can use bm.el. You can install bm.el from MELPA.

bm.el provides bm-toggle to highlight and unhighlight current line. bm.el also provides bm-bookmark-regexp which highlights only matched lines. And you can jump between highlighted lines by bm-previous and bm-next

Following is sample configuration of bm.el

(require 'bm)
(global-set-key (kbd "<f5>") 'bm-toggle)
(global-set-key (kbd "<f6>") 'bm-previous)
(global-set-key (kbd "<f7>") 'bm-next)
(global-set-key (kbd "<f8>") 'bm-bookmark-regexp)

Bookmark+ does what you are asking for. Use C-x p RET (by default) to set a bookmark at point. And you can configure the kind of highlighting you want for such bookmarks. This is similar to what bm.el offers (syohex's answer), but more flexible.

I just quickly wrote the following:

(defun find-overlays-specifying (prop pos)
  (let ((overlays (overlays-at pos))
        found)
    (while overlays
      (let ((overlay (car overlays)))
        (if (overlay-get overlay prop)
            (setq found (cons overlay found))))
      (setq overlays (cdr overlays)))
    found))

(defun highlight-or-dehighlight-line ()
  (interactive)
  (if (find-overlays-specifying
       'line-highlight-overlay-marker
       (line-beginning-position))
      (remove-overlays (line-beginning-position) (+ 1 (line-end-position)))
    (let ((overlay-highlight (make-overlay
                              (line-beginning-position)
                              (+ 1 (line-end-position)))))
        (overlay-put overlay-highlight 'face '(:background "lightgreen"))
        (overlay-put overlay-highlight 'line-highlight-overlay-marker t))))


(global-set-key [f8] 'highlight-or-dehighlight-line)

(Here find-overlays-specifying came from the manual page)

It will highlight current line, and when used again it will remove it.

Maybe the following could be useful as well: removing all your highlight from the buffer (could be dangerous, you might not want it if you highlight important things)

(defun remove-all-highlight ()
  (interactive)
  (remove-overlays (point-min) (point-max))
  )

(global-set-key [f9] 'remove-all-highlight)